The origins of the calendar

Origins of the calendar

Table of Contents

This is a page about the origins of our calendar: why 24 hour days, 7 day weeks, 30 and 31 day months, where seasons came from and how our calendar developed, correct millennium date, development of month names, moon names, Gregorian & Orthodox calendars.

Check to make custom calendars or get some free printable calendars here.


Days were kept track by observing the sun. Early days were counted as starting at sundown. The Jewish days are still reckoned this way. One cycle of dark followed by light was called a day. To avoid confusion, a day and a night is called nucthemera. They used unequal hours, there were 12 “hours” of night followed 12 “hours” of light. In summer the light “hours” where long and night “hours” were short and vice verse in the winter. So the 11 hour would be about our 8 PM EST in summer and 5 PM EST in winter. Some locations counted from sunrise, as India does.

Early people did not have long division so they loved number base that could have many ratios. Twelve has may divisors and so you can have rational (ratios) numbers for 1/2, 1/3, 1/4, and 1/6 of a day. In fact the Babylonian used a base 12 numbering system. It has come down to us today in the following forms: 10, 11, and 12 have their own unique names (not one – teen, etc.); we have words for units of 12 (dozen) and it’s next place ( 12*12 = gross) and even the following place (12*12*12 = great gross). The decimal number 1242 in base 12 is 876 – 8 gross, 7 dozen and 6.

Noon time was the easy to determine, the shadow would be in a line due north and south. This line due north and south through your position is called the meridian, and this is where we get the names for or A.M. (Ante Meridian) where the sun is before the meridian and P.M. (Post Meridian) where the is after the meridian. But as people needed to tell more than just noon, sunrise and sunset, they switched to equal hours so that clock speeds would not have to be changed twice a day nor would there be need of various lengths candles for determine the time at night. At same time we changed so midnight was the start of the day because noon, mid-day, was easy to determine but, they didn’t want to change the designation of the day in the middle of the work period so they used the time opposite — mid night. When we changed so the days started at midnight instead of sundown, the evening (from sundown to midnight) was referred to as the eve of the day that arrives at midnight.  This is keep the corresponding day to when the days started at sundown.  So if you were referring to the “old method” of keeping dates you would speak of the “eve” as belonging to the previous day.   For example, the eve of 25th is actually from sunset (the old start of the day) to midnight (start of the new day) of what we would now call the 24th.  This is where we get Christmas Eve, New Years Eve, etc.  Many people call all of December 31 “New Years Eve”, when only the time from sunset to midnight should be called Eve.
A.M. and P.M. do not apply to 12 o’clock because it is neither before or after the meridian, these are on the median. We must use the actual names for these times, noon and midnight.   At midnight the sun crosses the meridian as well as noon, but only people in the arctic zones see it.  Another problem, what date is midnight?  If I told you to meet me Sunday midnight, when would you be there?  Would you go late Saturday or go late Sunday?   The military fixed this problem by using a 24 hour clock (it just has 12 hours add to the time after noon).  It puts the minutes following the hours and calls it “hours.”  For example 1300  hours is 1:00PM. To meet some one at 0000 hours on Sunday, you leave late Saturday, and 24:00 (the military does not use 2400 hours, but say 2359 hours instead) you leave late Sunday to make the appointment.  That way there is no A.M. or P.M. to worry about.


Week Lengths

Most of the information in this section come from the essay “Moon over Babylon” by Dr. Isaac Asimov, the famous text books, popular science books, and science fiction books writer. This essay is collected in his book “Tragedy of the Moon.”  Of course, he’s a greater writer than me, if you are interested in the development of the week, read his essay.

Early in Babylonian times (circa 600 BC), the Babylonians worshiped the moon. As a celebration of every full moon, the start of the month (moon-th), they would have a worship service and everybody got off work that day and went to worship service where there was eating, drinking and dancing. This they called Sappatu and came to English as Sabbath.

The work productivity actual went up when they started this. The Jewish people became captives of the Babylonians and they celebrated the new moon. The Babylonians thought if one celebration of the moon is good two would be better. So they started another worship service at the new moon (in the middle of the month). This gave them two days a month off. After awhile production still went up so they tried a day off at the quarter moons also (the waxing and the waning Gibbons) — the time the moon appears as a half circle. In fact our English word Week comes from the Teutonic word for change — indicating the change of the phases of the moon.

The people really enjoyed having 4 days off every moon cycle, but they had a complaint. It was 7 days from one day off to the next but frequently it was 8 days and it was very hard to figure when it was 7 and when it was 8. Because, on the average, there are 29.530588 days per month. Their months were 29 or 30 days long based on observation of the visibility of the first crescent. It is hard to divide it, therefore, the days between were hard to compute. So they stopped having the Sabbath (as the 4 days off in month were then called) based on the moon, but instead had it every 7 days. The Babylonians liked the number 7, it was magic. That was the number of planets (as they knew) and the planets were gods and the days of the weeks were named after the gods. Also, if you took a circle (such as a coin), you could put 6 other same size circles (coins) around it and they would all touch the center and their neighbors. Now the circle represents a god, and a circle of gods around a god must mean a god of gods and therefore very powerful.

This could have come from the Egyptians who named the hours of the day after a god and the god of the first hour (at sunrise) was the primary god of the day. Since there are 7 planet gods and each ruled a day, then the Babylonians could not have no god to rule the 8 day because that would be chaotic. If they just started with the 7 god with 8 day and cycled through them, it would still be a 7 day week. So 7 days for the week became the standard.

The Jewish people who were captives (slaves) of the Babylonians got every 7 day off so they could worship also. So even when they escaped, they followed keeping (worshiping on) the Sabbath. In fact, they still use today a modified version of calendar the Babylonians had developed thousands of years ago.

At the time of Alexander the Great, the Greeks started using the Babylonian calendar, astronomy and astrology but used their gods names instead. The Romans also adapted the calendar from the Greeks and again using their names for the planets (their gods) (see “Week Names” below.) At first it was informal, brought back by the people stationed in the middle east.

And early Christian observed the Sabbath (Jewish worship), and also day after the Sabbath (by now considered the first day of the week) where they worshiped Christ. When the Roman empire converted to Christianity (324 C.E.), they took the first day of the week as the day of worship and a day off and the 7 days a week became official.

The second day off did not come about until after 1945 (after World War II) because there were too many workers.  The next day off was the Sabbath (our Saturday, the 7 day of the week) out of consideration of the Jews (who had suffered greatly during the war) and religious sect that followed Bible’s injunctions of remembering the Sabbath. Note: Monday is the first day of the week in only a few calendars.

Week Day Names

The Roman calendar had their days of the week named after the sun, moon & planets which were gods and subsequent versions of the calendar did the same. The ancients considered the sun and moon planets because they wandered (what the word “planets” meant) the sky. So we have Sunday (the Sun’s day), Monday (Moon’s day), Tuesday (Tiw’s day – Old Norse’s equivalent to planet and god of Mars), Wednesday (Woden’s day – Old Norse’s equivalent to Mercury), Thursday (Thor’s day – Old Norse’s equivalent to Jupiter), Friday (Frigg’s — also called Frica’s day – Old Norse’s equivalent to Venus), and Saturday (Saturn’s day). So we still have the days of the week named after the 7 “planets” that were known in ancient time.


The Egyptians and the Babylonians kept track of the year by observing the stars that were in the sky where the sun appeared (Helio Rising). They divided the year in to 12 parts because their number base was 12 or 60 and there was just a little more than 12 moon cycles in a year. They had the 12 parts each of 30 days and a partial month of 5 days. They divided the part of the sky where the sun rose into 12 parts to represent the 12 parts of year. They assigned the group of stars names and pictures, now called constellations, this we call the Zodiac. When the Sun first arose in a constellation, it was said to be entering the constellation. Basically, it is starting the next month of the year. However, the earth has a little wobble in its axis and this became inaccurate over the centuries. Now if we used the same zodiac and looked to see where the sun rises we would see that the sun has shifted 2 constellations. It is 2 months off. Yet astrologers still use the old calendar (zodiac) to cast horoscopes.

Our names for the constellations came from the Latin, who got it from the Arabs, who got it from the Babylonians. Many of the star names and the measurement system used to map the heaven still have their Arabic names. For example, Betelgeuse means “arm pit” (of the giant) referring to the what we call the shoulder of the constellation Orion.

Calendar Months

Month length

Early Calendars followed the moon — were we get our word for month. Each month of the year had a name that was dependent on the language and culture. We still have the moon names in our language — Harvest Moon is most obvious example. They had 29 to 30 days in a month, alternating between 29 and 30 days pretty much would keep the calendar in step with the moon phase. Actual average day in a month is 29.530588. Agrarian (food raising) cultures had a little problem with this because it would not keep in step with the seasons, 12 months were too short and 13 months were too long. Some non agrarian cultures just ignored the season and kept 12 moon cycles per year. This is how the Arabic countries keep their religious calendar and it only has 354 days in the year.

Agrarian cultures had to keep track of the seasons and wanted their calendars to do it for them but a pure lunar calendar was 11.2422 days too short. What they did was add an extra month to the year about every three years to get the calendar back in step with the seasons. So on the average, the year would come out about right and this is called a lunar – solar calendar. Most cultures with lunar calendars used this method. The Hebrew and Chinese calendars today still use this method. The Hebrew calendar adds a extra month to the middle of the year (as the seventh month) 7 times in a 19 year cycle. Also to keep the months in track with the lunar cycle, 3 of the months are variable (29 or 30 days in length) depending on the amount of adjustment needed. The Chinese calendar also adds a extra month when the error adds up be more than 1/2 of a month so the extra month can show up any time during the year. See “Calendars and Their History” for more information on the exact calculation

Early Roman calendar also added an extra month and it was controlled by the senate. They keep adding the extra month in a year so their term would run longer, you know how politicians behave. In 45 B.C.E. the calendar was very far off so Julius Caesar reformed the calendar and took it off the lunar cycle and put it on the solar cycle only. (See Caesarian on my miscellaneous page for an aside on Julius.) It started out as 5 months with 30 days and 6 months of 31 days with one month (February) having 29 and becoming 30 for leap year every 4 years. The beginning of each third month was the start of another season. But Augustus Caesar found the senate had counted the years incorrectly and had put a leap day every three years. Because of his correction the calendar, the senate wanted a month named for Augustus and the only one that didn’t have holidays already assigned was the month after one given to Julius (July). It is only a rumor that it only had 30 days in it, so to not be out done by Julius, the senate stole a day from February.

Month Names

This is a common explanation where the months got their name. In the Roman calendar, March was the first month of the year. It was named after Mars, the god of war, because that is when a war could be resumed. April came from the word meaning after or second. May was name for the goddess Miai, the goddess of increase. June was named for the goddess Juno, the queen of the gods and the goddess of marriage, I guess that even in Roman times there were a lot of June brides. Quintillis (Latin for 5) was renamed for Julius and became July. Sextillia (Latin for six) was renamed for Augustus and became August. September came from the word seven (septem), October from eight (octo), November from nine (novem), December from ten (decem). February was the last month of the year and it was named after Februariua, the ritual for atonement.

In 154 B.C.E., the Roman senate started meeting at the start of the month after December, which was named January after the god of Janus. Janus was a temple god which could look forward and backward at the same time. They started the calling January 1 the start of new year. They left the months named after numbers the same even though they were now 2 months later in the calendar. Note: Not all people who followed the Roman calendar used January 1 as the day the year number changed. Many cultures used their own determination and many, including the Americans, used the day of the spring equinox as the start of the year. It wasn’t until 1752, when America moved from the first of the year from Spring to January 1.

In 45 B.C.E., Julius reformed the calendar and the 7 month was named after him (July). Caesar Augustus took the 8 month in 8 C.E. rearranging the number of days in the months.

Even though the beginning of the year is January 1, because February can have a leap day added, it is easier in computing days in calendar by considering March as the first of the year and January and February as months at the end of the previous year. This is like the calendars before 153 B.C.E.

Moon Names & Once in a Blue Moon

Before we had our months names we named the moons of a year. This is were we got months — moon-ths. Our moon’s are associated with the time of the Full Moon, other cultures associate them with time of the New Moon. Here are the common moon names in the United States of America (combined eastern Amerind and European traditions) associated with their months:

January         Wolf Moon
February        Snow Moon
March           Worm Moon
April           Pink Moon
May             Flower Moon
June            Strawberry Moon or Rose Moon
July            Buck Moon
August          Sturgeon Moon or Green Corn Moon
September       Harvest Moon
October         Hunter's Moon
November        Beaver Moon
December        Cold Moon or Old Moon

But sometimes there are two moons in a month and there is a special name for the second month, the Blue Moon. This occurs because the lunar cycle is about 29.5 days and there is 30 or 31 days in a month except February. It is possible that February does not have a moon in it at all then January and March would have two moons. The Blue moon occurs about on the average about every 2.7 years. Whence comes the expression “once in a Blue Moon” indicating something that occurs infrequently.

In fact, Laraine Mesavage reports in The Moon Cycle and Spirit the Blue Moon is even more ancient and comes before our calendar when the seasons were tracked by the Zodiac and the Blue Moon referred to the second moon in a Zodiac constellation.



Many people think that the seasons are based on the Earth’s orbit of the Sun, but they are wrong. The Earth’s orbit around the sun is close to a year in length but not quite.

The seasons are because the Earth spins on it axis like a top. Like a spinning top it has a wobble, however this wobble is very small and take about 26,000 years for a revolution, so it can be ignored for our purposes. The poles of this spin are not perpendicular (straight up and down) to the Earth’s orbit around the Sun. This is what gives us our seasons. The poles point toward the star Polaris and as the Earth revolves around the Sun, it appears as if the Sun was moving up and down relative to the equator. When the poles points along the Earth – Sun radius it is either Summer or Winter. If the North Pole is pointing toward the Sun it is Summer in the Northern Hemisphere. If the poles are tangent to the orbit of the Earth, it is an equinox (either Vernal or Autumn).

Another way of looking at the seasons: Consider the angle formed by the Sun, Earth, and North Pole (for Northern Hemisphere). If the angle is a right angle, it is an equinox. If it is at the most obtuse (90 + 23.5 degrees), it is Winter. If it is the most acute (90 – 23.5 degrees), it is Summer.

In a geocentric (Earth centered) view, the Sun appears to circle the Earth daily in a plane (not an airplane but a flat surface). This plane changes its tilt in a north/south direction a little each day. When this plane matches our equator it is an equinox, either the Vernal or Autumn. When this plane is the highest (about 23.5 degrees above the equator), it is Summer Solstice and when it is the same below the equator it is Winter Solstice.


All agrarian (food raising) cultures has to observe the seasons; it means life or death. If there was a crop failure many people would starve to death. If they planted too early, a frost could kill the crops and they may not have enough seed saved over winter to plant another. If they planted too late, a frost could kill the crops before the produce was ready to harvest and there is no time to replant and harvest before winter. The priest were responsible to determine the solstices (Winter/Summer) and equinox (Vernal/Autumn). Many holidays (Holy Days — days special to the gods) are based upon these start of seasons. All these days have special observations and many of our celebrations are derived from them. Modern Wicca (old religion — sometimes called “white” witch craft) followers still celebrate these.

Century and Millennium

When is the first day of the new year?


“Everybody” knows that January 1, 2000 is the start of a new century, the 21st. Everybody is wrong!

Circa 525 C.E. the years started to be number from the date of Jesus’ birth as computed by the religious people. They called it A.D. (Anno Domini — the year of the lord) and some years latter, the years before are called B.C. (Before Christ). Before the change the years were numbered as such and such year of the reign of some king or emperor or from founding of Rome. The scholars made conversion tables for the old way of naming the years.

Things start numbering from 1, including years, not at 0, no matter what the computer scientist says. The day after December 31, 1 B.C. is January 1, 1 A.D. This means from 5 B.C. to 5 A.D. is 9 years not 10, there is a missing year, the year 0. Also the century 1 (the first 100 years) as commonly accepted starts at (Jan 1) year 1 and goes through the year (Dec 31) 100 as it would be figured in our current calendar. The second century (second 100 years) is from year 101 through 200. Etc. Also the first 1000 years (millennium) would be 1 AD to 1000 AD, 2nd millennium from 100 1 AD to 2000 AD and the 3rd millennium from 2001 AD to 3000 AD.  Also the 20th century is from year January 1, 1901 to December 31, 2000; and 21st century is going to be from year 2001 to 2100. When somebody says January 1, 2000 is the first day of the 21st century or beginning of 3rd millennium, tell them it is beginning of the last year of the 20th century or the last year of the second millennium. It can’t even be called the beginning of the 20th hundred year because that is what the word century means.

Latter scholars recomputed the date Christ could be born and found they had set the date of Christ Birth by at least 4 years too late and many believe it may be 6 years late. By then most people had changed their calendars and so the error is just ignored. The Bible reports the Magi arrived in Jerusalem to discus with King Herod, the star announcing the birth of a new king and they and come to pay their respects. Well, Herod died 4 B.C. by our method of computing dates. The Magi had to have travel time to get to Jerusalem base on the time the Magi saw the star, Herod killed all Jewish children that were 2 years or younger. So if we allow 1 year for travel (half age of the children) and Herod dying in 4 B.C., the birth of Christ has to be at least 5 BC. Normally, it is reported that Christ was born about 4 BC. It is strange that the computed date of Christ’s birth is 4 B.C. (4 years Before Christ)! Anglican Archbishop James Ussher in 1650 stated that Christ was born on October 23, 4 BC.

To avoid the confusion with B.C. and A.D. when A.D. actually is incorrectly identifying the starting date, people are starting to use C.E. (Common Era) and B.C.E. (Before Common Era) to refer to the date of the start of our calendar.

But if we go by latest day for the Birth of Christ, the first century is from 5 BC to 96 AD, and the 21th century starts on January 1, 1997. If we consider that Christ was born in the fall (the shepherds were in the fields watching their flocks), the 21th century will be in the fall of 1997, which is the latest the 21th century could start. If fact if we count the years, the first millennium will end October 22, 1997 and the new, second, millennium began on October 23, 1997.

Of course, not a lot of people care about the truth and are excited about seeing the three zeros at the end of the year number and want to have a big celebration about it, therefore they will want to call it something special. So even with all the corrections and notifications of the truth, I still expect to hear that January 1, 2000 is the beginning of the new (21st) Century and the beginning of the new (3rd) millennium.

Modern Calendars

The calendar, as reformed by Julius Caesar, lasted many centuries with minor changes such as renaming the holidays, change so date from fixed date rather than a ruler. But there was a problem. The way the calendar was set up, it had 365.25 days per year average (the .25 from a leap day every 4 years). The actual days in a year is 365.2421896698 -.00000000615359*T – 7.29E-10*T*T + 2.64E-10*T*T*T (Where T is (JD – 2451545.0)/36525 and JD is the Julian Day number — the number of days since January 1, 4713 B.C.) Source: Calendars and their history. The epoch (point of calculation as shown by JD – 2451545.0) is January 1, 2000. This caused the calendar to have about 1/4 extra days every century and the seasons were getting latter in the calendar all the time.

Gregorian Calendar

So Pope Gregory (in 1582) adopted a new calendar that corrected it by 1/4 days per century. His calendar would not have a leap year on the years ending in 00 unless it was was divisible by 400. This gives 365.2425 days on the average (.24 is leap year every 4 years except for the year ending in 00 and .0025 is leap day every 4th year ending in 00. This is the current calendar we use — called the Gregorian calendar. To adjust so that Easter occurred closer spring equinox as they were in the olden days (325 C.E.), he had 10 days deleted from the calendar the day after October 4, 1582 would be October 15, 1582. The whole purpose was to make the spring occur on March 21, or as close as possible.

The countries that were predominate Catholic adopted the calendar. The protestant countries adopted it later. England (and United States of America as her colony) adopted the calendar in 1752 and to get the calendars to agree, September 2, 1752 was followed by September 14, 1752 (missing 11 days).

Orthodox Calendar

When the Pan Orthodox Churches counsel met (Russian, Greek, Serbian, Romanian, etc.) in October of 1923, they proposed a new calendar that match the Gregorian calendar except they compute the leap centuries differently. They said that if the year ended in 00 was divided by 9 has a 2 or a 6 remainder then it will be a leap century otherwise not. This gives 365.242222… (the .002222… is 2/9 days per century) as the average number of days in a year. As you can see this is much closer to the actual days in a year than the Gregorian. So in 2800, this calendar will not have a leap year and the rest of the world will unless the calendar is changed again otherwise it is the same as the Gregorian. Another calendar enthusiast, Richard McCarty, has provided a translation explaining the decision. The following accepted the new calendar: Greek, Romanian, Bulgarian, Oecumenic Constantinople and Alexandrian Church in 1924 or afterwards. The following Orthodox churches accepted neither new calendar but still reckon their feasts according to Julian calendar: Russian, Serbian and Georgian Church, Patriachate of Jerusalem, Russian Orthodox Church Abroad and all but one monasteries in Mount Athos. Their religious calendar is 13 days behind the Gregorian. This has caused a split in the Orthodox community because of the breaking of “the unity of prayer.”

Rather than to divide by 9 to find a reminder when dividing 9, you can use the math short cut called “casting out nines.” You determine the remainder by adding up the digits (you can ignore 0 and 9) and if the sum of digits is more than 9 add the digits in the sum until you have a single digit. If that digit is 0 or 9, there is no remainder. For example, the remainder of dividing 808 by 9 is 7 (8 + 8 = 16 — since has 2 digits repeat on it, 1 + 6 = 7). Another example: 777 divided by 9 has a remainder of 3 (7+7=14 — over 9, 1+4=5, 5+7=12 — over 9, 1+2=3).


One big challenge to Orthodox calendar by calendar enthusiast on the net says the Gregorian calendar is to have March 21 fall on the Spring (Vernal) equinox not to precisely track the year. Simon’s Calendar page say the Gregorian calendar does this better.

You may think that this would be the most accurate calendar. The Mayan is more accurate but extremely complex. It had cycles of 13, 20, 260, 365, 18,890 days and method of handling long dates of 400 years, 5,125 years, and 64 million years. The Mayan calendar even takes in the 26,000 year wobble of the axis. Their calendar cycle will end on March 21, 2012, the end of the age. The “new – age” people are making a big deal of this. For more information see Prehistoric Calendars, it has a great drawing of the Mayan Calendar stone, explanations, pronunciation of the dates and a date converter. Also see The Classic Maya Calendar and Day Numbering System. One of the more strange things (at least to current observation) is that it tracked two separate events at the same time. The 260 day “year” followed the rising of Venus, and of course the 365 days followed the tropical year. It appears the Mayans knew the 365 days of the year was inaccurate (closer to 365.242?) but they didn’t care. The other aspects of the calendar were more important that tracking the seasons. It could be they were from the tropics were the seasons meant less.


Get every new post delivered to your Inbox.