8th to 4th Century B.C. Calendar Changes

Most peoples all over the world either modified or discarded their old 360-day calendars starting sometime in the eighth century B.C. 1   Many of those calendars had been in use for the greater part of a millennium. In many places, month lengths immediately after that change were not fixed, but were based instead upon observation of the sky.
Priest-astronomers were assigned the duty of declaring when a new month began—it was usually said to have started at the first sighting of a new moon. 2   Month length at that time was simply the number of days that passed from one new lunar crescent to the next.

A 360-day calendar was used in Rome for a few years following the time Romulus added January and February to their calendar. Shortly after Romulus’ death a priest observed the sky and called out when there was a new moon and therefore a new month. For centuries afterward Romans referred to the first day of each new month as Kalends 3   or Calends from their word calare (to announce solemnly, to call out). Obviously, the word calendar was derived from this custom.
This practice of starting a month at the first sighting of a new moon was observed not only by Romans but by Celts and Germans in Europe and by Babylonians and Hebrews in the Lavant. 4   All of these peoples began their month when a young crescent was first seen in the sky. This is still done for the Islamic Calendar, but a new moon’s date is calculated for traditional lunar calendars that are currently used in China and India. 5
During the period when month lengths were not fixed, new moons were usually sighted after either 29 or 30 days. If clouds obscured vision on the thirtieth day, a new month was declared to have begun.
When month lengths were identical with lunations, only those that lasted 30 days were considered to be normal. This was probably because all months had previously been 30 days for such a long period of time.
In many nations, months that consisted of 30 days were considered to be “full;” those that lasted only 29 days were said to be “hollow.” 6   Months containing 30 days were also called “full” in Babylon, but those containing 29 were deemed to be “defective.”
After month lengths in the Celtic Calendar became fixed, those that had been given 30 days were termed “matos” (lucky) and those given 29 days “anmatos” (unlucky). 7   This notion has not died out. Even in our time, months of 30 days in the Hebrew Calendar are called “full” and those with 29 are deemed to be “deficient.” 8
In addition to their declaring the beginning of each month based upon a sighting of the new moon, priest-astronomers were also charged with pinpointing the start of a year. You might want to read about Sirius and the Early Egyptians for their method of determining when the new year began.
By observing the movement of Sirius, Egyptians came to grips with the fact that the year was longer than their venerable 360-day calendar. This resulted in a change to their method of approximating year length that had been in use for nearly a millennium. But it also caused them to wonder where the additional days came from. In order to account for these additional days, Egyptians created a myth about their sky-god, Nut. 9   It is related at Moon Myths.
Usually at a time later than the mid-eighth century B.C., many other peoples who had previously considered the year to be 360 days in length reluctantly changed to a calendar of twelve fixed 30-day months, but added “epagomenal days” to follow those months. Like the previously observed 29-day months, these additional days were considered to be very unlucky or unpropitious.
Calendars described in books of Enoch and Jubilees were 364 days in length. Cave 4 from a Dead Sea Scrolls excavation at Qumran produced eleven fragments of calendar texts which revealed that the Qumran community’s year was also 364 days. It “consisted of twelve months of thirty days each, plus four extra days added to each of the four seasons.” 10   No mention of intercalation beyond those four days appears in Jubilees or Enoch and was never found in Dead Sea Scrolls. In fact, Enoch claimed that the 364-day calendar year was perfect and should not be tampered with.
During the reign of the Babylonian king Nabonasser (traditionally dated between 747 and 734 B.C.) priest-astronomers in that country discontinued their practice of looking for the new moon in order to name the beginning of a month. Instead, they returned to a fixed-length calendar that had 12 months of 30 days each, but with five days added at the end. 11
Two eastern Mediterranean peoples who did not embrace Islam were early Christians in upper Egypt, whom we now call Copts, and their neighbors to the south, the Ethiopians. Probably because they were surrounded by Islamic peoples, Coptic and Ethiopian churches never adopted the Western calendar. Instead, these two isolated pockets of Christianity continued to use the old 360-day calendar.
These two calendars are identical except for year number. Both of them observe three 365 day years followed by one 366 day year. Their years are divided into 12 months of 30 days each, and the extra five or six days are added after the twelfth month.
The Ethiopian Orthodox Church still schedules its religious observances over twelve 30-day months followed by a “little month” of either five or six days beginning on the Western calendar’s September 6. 12
Zoroastrians use a calendar of 365 days whose era began at 389 B.C., the year of Zoroaster’s birth. It consists of twelve 30-day months with five “Gatha days” added to the final month of the year. Each of the thirty days as well as each of the Gatha days has its own name. They are referred to by that name just as we speak of a day by its number in the month. Beginning in 1906 of the Common Era, some modern Zoroastrians adopted the practice of adding an additional day every four years. 13
One of Alexander the Great’s generals, Seleucus Necator, founded (early in the 4th century B.C.) an empire that stretched from Asia Minor to India. He established a new calendar that was essentially the same as one that had been used for some time in Syria. It contained twelve months of 30 days each and an extra five days at the year’s end. Every fourth year an additional day for a total of six days were added at the end of the year. 14
In Persia under the Sassanids, and in Armenia and Cappadocia the official system of time-reckoning was twelve months of 30 days followed by five more days at the end of the year. The Sassanian year of twelve 30-day months (plus 5 days) was kept adjusted to the seasons by observing an extra month every 120 years. 15
The 360-day calendar in Persia has been referred to as the “Old-Avestan” but the name was changed to “Young-Avestan” after five Gatha days were added at the end of each year. This happened between 510 and 487 B.C. depending on which historian is correct. 16
Mayan people in Central America also had a 360-day calendar-year tradition. Within it were eighteen 20-day periods. By the 4th century B.C., five days had been added to the end of those calendar years, but were not included as part of the original final period. Those five days were considered to be unlucky.
Mayans also developed another calendar around 500 BCE to be used for religious purposes. With it they were able to take a different approach than either Europeans or Asians. They merely reduced the number of 20-day periods in the old calendar from eighteen to thirteen per cycle, which resulted in 260-day calendar years. 17
Worldwide calendar changes described on this page occurred during a period of religious and other global social changes known as the “Axial Age.” 18

REFERENCES:
(1) Parise, Frank, Editor. The Book of Calendars.
New York: Facts On File, Inc. p. 126.
(2) Encyclopaedia Britannica Babylonian Calendars
(3) Blackburn, B & Holford-Strevens, L.
The Oxford Companion to the Year:
An Exploration of Calendar Customs and Time-Reckoning

Oxford: Oxford University Press, 1999. p. 672.
(4) Bickerman, Elias Joseph. Chronology of the Ancient World.
Ithica, N.Y.: Cornell University Press, 1968. p. 17.
(5) Doggett, L.E. Calendars.
P. Kenneth Seidelmann, Editor.
Sausalito, California   94965: University Science Books.
(6) Blackburn, B & Holford-Strevens, L. op. cit., p. 880-881.
(7) Celtic Religion
30-day months were thought of as “good” and
29-day months “bad” in the Coligny Calendar.
(8) Doggett, L.E. op. cit.
(9) Goudsmit, Samuel A. Time.
New York: Time Incorporated. 1966. p. 69.
(10) Vermes, Geza. The Complete Dead Sea Scrolls in English.
New York: Penguin. 1997 p. 335.
(11) Parise op. cit.
(12) Ethiopian Calendar (See Ethiopian calendar year at this site)
(13) Vispi Homi Bulsara. Zoroastrian Calendar
(14) Parise op. cit., p. 44.
(15) Christensen, A.  L’Iran des Sassanides. 1944. p. 168.
(16) Taqizadeh, S.H. Old Iranian Calendars
(17) The Mayan Calendar
(18) Koeller, David W. The “Axial Age”