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The Jewish Calendar, a History

In the Torah it is written:

Wayyiqra’ [Leviticus] 23:4 These are the set times of the YHWH, holy convocations, which you shall proclaim at their appointed times.

The evident meaning of this verse is that God fixed festivals and holy days for us, and commanded us to observe them at the appointed times, on the dates determined in the Torah. In order to fulfill this obligation correctly, we must be able to determine the exact day of the beginning of each month, which itself is sanctified. This day is the basis on which the dates of the festivals and holy days are calculated.

Originally, the first day of the months was fixed by all of Israel using visual evidence, that is, a sighting of the appearance of the new moon. This is based on the text:

Bere’shit [Genesis] 1:14 God said, "Let there be lights in the expanse of the sky to separate day from night; they shall serve as signs for the set times, the days and the years.

The fact that all Israel, including the Rabbanites, sanctified the beginning of each month with the appearance of the new moon is attested in their Mishnah, which alleges that God showed Moses the new moon and said to him: “See this, and sanctify it.” Similarly, there is the discussion in the Talmud between Rabbi ‘Aqiva and Rabban Yehoshu’a‘ concerning the time of the Day of Atonement. Masekhet Rosh Hashannah 2:9 details the invention of Rabbi ‘Aqiva who is trying to justify what Gamali’el had done when he commanded Yehoshu’a‘ to come on Yom Kippur with his money and his staff in his hand and his shoes on his feet (i.e., to be Mehalel [profaning] Yom Kippur) in order to force him to follow his version of the calendar (which was based on provenly false witnesses). ‘Aqiva here provides a way for Yehoshu’a‘ to do what he has been commanded. ‘Aqiva contends, by changing the wording of the text, that the text states: “You have no holidays except for the ones that you {the rabbis} proclaim”, this is as opposed to what the text says in context “that you shall proclaim at their appointed times”.

If all Israel determined the calendar by the visual sighting of the appearance of the new moon, what happened? Why, today, do the Rabbanites have a fixed calendar? How does their calendar work? How did their calendar come to be? How is it different from the Karaite calendar? Which calendar is correct?

Like many things Rabbanite, the Rabbanite calendar finds it origins in Babylon. The beginning of the month in the Babylonian calendar was determined by the direct observation by priests of the young crescent moon at sunset after the astronomical New Moon. If weather prevented the observation of the crescent, the Babylonians would begin the new month anyway after a set number of days. Each month was given a conventional length, alternating 30 days and 29 days. See the table below for more information:

The Babylonian Calendar




1. Nisannu 30 7. Tashritu 30
2. Aiyaru 29 8. Arakhsamna 29
3. Simannu 30 9. Kislimu 30
4. Du'uzu 29 10. D.abitu 29
5. Abu 30 11. Sabadu 30
6. Ululu I 29 12. Addaru I 29
6. Ululu II 29 12. Addaru II 30









The Babylonian calendar, however, was also subject to corrections based on a 19 year cycle. The 19 year cycle adjusts lunar months to the solar year. However, given the fact that the Babylonian New Year was supposed to be the first New Moon after the Vernal Equinox, the calendar would run slow and the cycle would need correction. The correction can be accomplished simply by delaying every single intercalation a whole year.

The 19 year cycle is as follows. Adding 7 months every 19 years approximates the solar year with 235 lunar months. That is the most mathematically accurate cycle for a luni-solar calendar. It would give, using the mean value of the synodic month (29.530588 days), a year of 365.2467463 days long. This is called the “Metonic” year, after the Greek astronomer who described the cycle, although the Babylonians discovered it first. The mean solar (tropical) year is 365.24219878 days long. The calendar thus has two problems: (1) It is less accurate than the Gregorian (365.2425) and must in the long run make provision for correction - it is off a day every 219 years against the sun. (2) The calendar cannot be corrected for the sun by subtracting a day every 219 years or so, because this would put it out of synchronization with the moon. A luni-solar calendar must regulate its lunar side with days and its solar side by its addition of months. The solar side therefore must be corrected by modifying the 19 year cycle. This may be done by delaying an intercalation every 342 years (18 cycles). By such delays, the calendar would lose an entire month after 6498 years, which reduces the Metonic year to 365.2422018 days, accurate to a day in 336,700 years.

As with many things Rabbanite (see Rabbanism and Zoroastrianism), the current Rabbanite calendar essentially is the Babylonian Calendar. The Rabbanite calendar retains not only the Babylonian Month names (e.g. Nisan for Nisannu) but also the Babylonian 19 year cycle, and each month is given a conventional length, alternating 30 days and 29 days. The official adoption of this calendar was the reform effected by Hillel II in the 4th century. However, the current calendar dates from the 9th or 10th centuries, when the complete system was apparently formulated. The 19 year cycle is the only true cycle in the Rabbanite calendar, since the method of adding days depends on the mean value of the synodic month and does not produce a repetition of dates within any significant length of time. Compare the Rabbanite calendar with the Babylonian calendar (at bottom).

In contrast with the Rabbanite calendar, the Torah calendar depends solely on visual evidence for the sighting of the moon to determine the start/end of the Biblical months. Furthermore, the true, Biblical calendar does not have month names - rather months are referred to by their number (i.e., first month, second month, etc.,), as can be attested to by the following passages:

Shemot [Exodus] 12:2 “This month [shall be] your beginning of months; it [shall be] the first month of the year to you”.

Shemot [Exodus] 16:1 And they journeyed from Elim, and all the congregation of the children of Israel came to the Wilderness of Sin, which is between Elim and Sinai, on the fifteenth day of the second month after they departed from the land of Egypt.

Wayyiqra’ [Leviticus] 16:29 . “ [This] shall be a statute forever for you: In the seventh month, on the tenth [day] of the month, you shall afflict your souls, and do no work at all, [whether] a native of your own country or a stranger who dwells among you”.

Wayyiqra’ [Leviticus] 23:34 “Speak to the children of Israel, saying: ‘The fifteenth day of this seventh month [shall be] the Feast of Arbor-Booths [for] seven days to the LORD’”.

Not once in the Torah is a month given a proper name.

Later on in the TaNaKh, names for months are recorded in the book of Melakhim [Kings] (Melakhim Alef I Kings 6:1, 6:37-38, 8:2). The names recorded are Yerah Bul [month of produce], Yerah Ziw [month of splendor], and HaEitanim [month of the powers (of nature)]. These names only appear in the book of Melakhim [Kings], as listed above; and all of these passages may be found in the account of the construction of the Temple. Many scholars are of the opinion that names are of Phoenician/Canaanite origin. There are many reasons for this. First, the text translates these foreign month names into Torah month names - i.e., “in the month Ziw, that is to say the second month”, etc. Furthermore, Phoenicians (the northern the Canaanites) were actively engaged in the building of the Temple (Melakhim Alef [I Kings 5).

One question remains, “How do we know these are Phoenician names?” This is confirmed by the fact that one of the names, Yerah Bul, has been found in Phoenician and Canaanite inscriptions. If we know that Yerah Bul is a Phoenician name then most certainly, Yerah Ziw and Yerah HaEtanim also are.

What then is the true biblical calendar? The true biblical calendar is determined by visual evidence - the sighting of the new moon; it has no fixed number of days. The true biblical calendar has no month names - each month is referred to by its number. The true biblical calendar begins with the month of Passover, which is determined by the sighting of Aviv (ripening ears of barley at a particular stage in their development) in the land of Israel.* The new year does not occur at the time of Yom Teru‘ah, which Rabbanites refer to erroneously as Rosh HaShanah (Exodus 12).



* A leap year occurs when the Aviv is not sighted until a month after the time when the new year would normally begin.

**In counting the year from Nisan, Adar is the month that is doubled, but in counting the year from Tashritu, Ululu would be doubled.