The Hebrew calendar (Hebrew: הלוח העברי‎) or Jewish calendar is the annual calendar used in Judaism. It determines the dates of the Jewish holidays, the appropriate Torah portions for public reading, Yahrzeits (the date to commemorate the death of a relative), and the specific daily Psalms which some customarily read. Two major forms of the calendar have been used: an observational form used prior to the destruction of the Second Temple in 70 CE, and based on witnesses observing the phase of the moon, and a rule-based form first fully described by Maimonides in 1178 CE, which was adopted over a transition period between 70 and 1178.

The "modern" form is a rule-based lunisolar calendar, akin to the Chinese calendar, measuring months defined in lunar cycles as well as years measured in solar cycles, and distinct from the purely lunar Islamic calendar and the almost entirely solar Gregorian calendar. Because of the roughly 11 day difference between twelve lunar months and one solar year, the calendar repeats in a Metonic 19-year cycle of 235 lunar months, with an extra lunar month added once every two or three years, for a total of seven times every nineteen years. As the Hebrew calendar was developed in the region east of the Mediterranean Sea, references to seasons reflect the times and climate of the Northern Hemisphere.

Biblical period

Jews have been using a lunisolar calendar since Biblical times. The first commandment the Jewish People received as a nation was the commandment to determine the New Moon. The beginning of Exodus Chapter 12 says "This month (Nissan) is for you the first of months.". The months were originally referred to in the Bible by number rather than name. Only four pre-exilic month names appear in the Tanakh (the Hebrew Bible): Aviv (first; literally "Spring", but originally probably meant the ripening of barley), Ziv (second; literally "Light"), Ethanim (seventh; literally "Strong" in plural, perhaps referring to strong rains), and Bul (eighth), and all are Canaanite names, and at least two are Phoenician (Northern Canaanite). It is possible that all of the months were initially identifiable by native Jewish numbers or foreign Canaanite/Phoenician names, but other names do not appear in the Bible.

Furthermore, because solar years cannot be divided evenly into lunar months, an extra embolismic or intercalary month must be added to prevent the starting date of the lunar cycles from "drifting" away from the Spring, although there is no direct mention of this in the Bible. There are hints, however, that the first month (today's Nissan) had always started only following the ripening of barley; according to some traditions, in case the barley had not ripened yet, a second last month would have been added. Only much later was a systematic method for adding a second last month, today's Adar I, adopted.

Babylonian exile

During the Babylonian exile, immediately after 586 BCE, Jews adopted Babylonian names for the months, and some sects, such as the Essenes, used a solar calendar during the last two centuries BCE. The Babylonian calendar was the direct descendant of the Sumerian calendar.

Names and lengths of the months

Hebrew names of the months with their Babylonian analogs
Number Hebrew name Length Babylonian analog Notes
1 Nisan / Nissan 30 days Nisanu called Aviv in the Tanakh
2 Iyar 29 days Ayaru called Ziv in the Tanakh
3 Sivan 30 days Simanu
4 Tammuz 29 days Du'uzu
5 Av 30 days Abu
6 Elul 29 days Ululu
7 Tishrei 30 days Tashritu called Eitanim in the Tanakh
8 Cheshvan 29 or 30 days Arakhsamna also spelled Heshvan; called Bul in the Tanakh
9 Kislev 30 or 29 days Kislimu also spelled Chislev
10 Tevet 29 days Tebetu
11 Shevat 30 days Shabatu
12 Adar I 30 days Adaru Only in leap years
13 Adar / Adar II 29 days Adaru

During leap years Adar I (or Adar Aleph — "first Adar") is considered to be the extra month, and has 30 days. Adar II (or Adar Bet — "second Adar") is the "real" Adar, and has 29 days as usual. For example, in a leap year, the holiday of Purim is in Adar II, not Adar I.

Second Temple era

In Second Temple times, the beginning of each lunar month was decided by two eyewitnesses testifying to having seen the new crescent moon. Patriarch Gamaliel II (c. 100) compared these accounts to drawings of the lunar phases. According to tradition, these observations were compared against calculations made by the main Jewish court, the Sanhedrin. Whether or not an embolismic month (a second Adar) was needed depended on the condition of roads used by families to come to Jerusalem for Passover, on an adequate number of lambs which were to be sacrificed at the Temple, and on the earing of barley needed for first fruits.

Once decided, the beginning of each Hebrew month was first announced to other communities by signal fires lit on mountaintops, but after the Samaritans and Boethusaeans began to light false fires, a shaliach was sent. The inability of the shaliach to reach communities outside Israel within one day, led outlying communities to celebrate scriptural festivals for two days rather than for one, observing the second feast-day of the Jewish diaspora because of uncertainty of whether the previous month was 29 or 30 days.

From the times of the Amoraim (third to fifth centuries), calculations were increasingly used, for example by Samuel the astronomer, who stated during the first half of the third century that the year contained 365 ¼ days, and by "calculators of the calendar" circa 300. Jose, an Amora who lived during the second half of the fourth century, stated that the feast of Purim, 14 Adar, could not fall on a Sabbath nor a Monday, lest 10 Tishri (Yom Kippur) fall on a Friday or a Sunday. This indicates a fixed number of days in all months from Adar to Elul, also implying that the extra month was already a second Adar added before the regular Adar.

Roman Era

The Jewish-Roman wars of 66–73, 115–117, and 132–135 caused major disruptions in Jewish life, also disrupting the calendar. During the third and fourth centuries, Christian sources describe the use of eight, nineteen, and 84 year lunisolar cycles by Jews, all linked to the civil calendars used by various communities of Diaspora Jews, which were effectively isolated from Levant Jews and their calendar. Some assigned major Jewish festivals to fixed solar calendar dates, whereas others used epacts to specify how many days before major civil solar dates Jewish lunar months were to begin.

Alexandrian Jewish calendar

The Ethiopic Christian computus (used to calculate Easter) describes in detail a Jewish calendar which must have been used by Alexandrian Jews near the end of the third century. These Jews formed a relatively new community in the aftermath of the annihilation (by murder or enslavement) of all Alexandrian Jews by Emperor Trajan at the end of the 115–117 Kitos War. Their calendar used the same epacts in nineteen year cycles that were to become canonical in the Easter computus used by almost all medieval Christians, both those in the Latin West and the Hellenist East. Only those churches beyond the eastern border of the Byzantine Empire differed, changing one epact every nineteen years, causing four Easters every 532 years to differ.

Transition period - period lol

The period between 70 and 1178 was a transition period between the two forms, with the gradual adoption of more and more of the rules characteristic of the modern form. Except for the modern year number, the modern rules reached their final form before 820 or 921, with some uncertainty regarding when. The modern Hebrew calendar cannot be used to calculate Biblical dates because new moon dates may be in error by up to four days, and months may be in error by up to four months. The latter accounts for the irregular intercalation (adding of extra months) that was performed in three successive years in the early second century, according to the Talmud.

Evidence for adoption of the modern rules

A popular tradition, first mentioned by Hai Gaon (d.1038), holds that the modern continuous calendar was formerly a secret known only to a council of sages or "calendar committee," and that Patriarch Hillel II revealed it in 359 due to Christian persecution. However, the Talmud, which did not reach its final form until c. 500, does not mention the continuous calendar or even anything as mundane as either the nineteen-year cycle or the length of any month, despite discussing the characteristics of earlier calendars.

Furthermore, Jewish dates during post-Talmudic times (specifically in 506 and 776) are impossible using modern rules, and all evidence points to the development of the arithmetic rules of the modern calendar in Babylonia during the times of the Geonim (seventh to eighth centuries), with most of the modern rules in place by about 820, according to the Muslim astronomer Muḥammad ibn Mūsā al-Ḵwārizmī. One notable difference was the date of the epoch (the fixed reference point at the beginning of year 1), which at that time was identified as one year later than the epoch of the modern calendar.

Controversy over the Passover of 4682 AM

The Babylonian rules required the delay of the first day of Tishri when the new moon occurred after noon.

In 921, Aaron ben Meir, a person otherwise unknown, sought to return the authority for the calendar to the Land of Israel by asserting that the first day of Tishri should be the day of the new moon unless the new moon occurred more than 642 parts (35 2/3 minutes, where a "part" is 1/1080 of an hour or 1/18 of a minute or 3 1/3 seconds) after noon, when it should be delayed by one or two days. He may have been asserting that the calendar should be run according to Jerusalem time, not Babylonian. Local time on the Babylonian meridian was presumably 642 parts later than on the meridian of Jerusalem.

An alternative explanation for the 642 parts is that if Creation occurred in the Autumn, to coincide with the observance of Rosh Hashana (which marks the changing of the calendar year), the calculated time of New Moon during the six days of creation was on Friday at 14 hours exactly (counting from the day starting at 6pm the previous evening). However, if Creation actually occurred six months earlier, in the Spring, the new moon would have occurred at 9 hours and 642 parts on Wednesday. Ben Meir may thus have believed, along with many earlier Jewish scholars, that creation occurred in Spring and the calendar rules had been adjusted by 642 parts to fit in with an Autumn date;

In any event he was opposed by Saadiah Gaon. Only a few Jewish communities accepted ben Meir's opinion, and even these soon rejected it. Accounts of the controversy show that all of the rules of the modern calendar (except for the epoch) were in place before 921.

In 1000, the Muslim chronologist al-Biruni also described all of the modern rules except that he specified three different epochs used by various Jewish communities being one, two, or three years later than the modern epoch. Finally, in 1178 Maimonides described all of the modern rules, including the modern epochal year.

When does the year begin?

According to the Mishnah (Rosh Hashanah 1:1), there are four days which mark the beginning of the year, for different purposes:

  • Months are numbered from Nisan, reflecting the injunction in Exodus 12:2, "This month shall be to you the beginning of months," and Nisan marks the new year for civil purposes.
  • The day which is most often referred to as the "New Year" is observed on the first of Tishri, when the year number increases by 1 and the formal new year festival Rosh Hashana is celebrated. It also marks the new year for certain agricultural laws.
  • The month of Elul is the New Year for certain matters connected with animals.
  • Tu Bishvat ("the 15th of Shevat (ט"ו בשבט),") marks the new year for trees.
  • There may be an echo here of a controversy in the Talmud about whether the world was created in Tishri or Nisan; it was decided that the answer is Tishri.


The average length of the month assumed by the calendar is correct within a fraction of a second (although individual months may be a few hours longer or shorter than average). There will thus be no significant errors from this source for a very long time. However, the assumption that 19 tropical years exactly equal 235 months is wrong, so the average length of a 19 year cycle is too long (compared with 19 tropical years) by about 0.088 days or just over 2 hours. Thus on average the calendar gets further out of step with the tropical year by roughly one day in 216 years. If the intention of the calendar is that Pesach should fall on the first full moon after the vernal equinox, this is still the case in most years. However, at present three times in 19 years Pesach is a month late by this criterion (as in 2005). Clearly, this problem will get worse over time and if the calendar is not amended, Pesach and the other festivals will progress through a complete cycle of seasons in about 79,000 years.

As the 19 year cycle (and indeed all aspects of the calendar) is part of codified Jewish law, it would only be possible to amend it if a Sanhedrin could be convened. It is traditionally assumed that this will take place upon the coming of the Messiah, which will mark the beginning of the era of redemption according to Jewish belief. Theoretically, if Jewish law could be modified, one solution would be to replace the 19-year cycle with a 334-year cycle of 4131 lunations. This cycle has an error of only one day in about 11,500 years. However, this would be impossibly cumbersome in practice. Further, no such mathematically fixed rule could be valid in perpetuity, because the lengths of both the month and tropical year are slowly changing. Another possibility would be to calculate the approximate time of the vernal equinox and have a leap year if and only if Pesach would otherwise start before the vernal equinox. Similar ideas are used in the Chinese calendar and some Indian calendars.


  • The Code of Maimonides (Mishneh Torah), Book Three, Treatise Eight: Sanctification of the New Moon. Translated by Solomon Gandz. Yale Judaica Series Volume XI, Yale University Press, New Haven, Conn., 1956.
  • Ernest Wiesenberg. "Appendix: Addenda and Corrigenda to Treatise VIII". The Code of Maimonides (Mishneh Torah), Book Three: The Book of Seasons. Yale Judaica Series Volume XIV, Yale University Press, New Haven, Conn., 1961. pp.557-602.
  • Samuel Poznanski. "Calendar (Jewish)". Encylopædia of Religion and Ethics, 1911.
  • F.H. Woods. "Calendar (Hebrew)", Encylopædia of Religion and Ethics, 1911.
  • Sherrard Beaumont Burnaby. Elements of the Jewish and Muhammadan Calendars. George Bell and Sons, London, 1901.
  • W.H. Feldman. Rabbinical Mathematics and Astronomy,3rd edition, Sepher-Hermon Press, 1978.
  • Otto Neugebauer. Ethiopic astronomy and computus. Österreichische Akademie der Wissenschaften, philosophisch-historische klasse, sitzungsberichte 347. Vienna, 1979.
  • Ari Belenkiy. "A Unique Feature of the Jewish Calendar — Dehiyot". Culture and Cosmos 6 (2002) 3-22.
  • Arthur Spier. The Comprehensive Hebrew Calendar. Feldheim, 1986.
  • L.A. Resnikoff. "Jewish calendar calculations", Scripta Mathematica 9 (1943) 191-195, 274-277.
  • Edward M. Reingold and Nachum Dershowitz. Calendrical Calculations: The Millennium Edition. Cambridge University Press; 2 edition (2001). ISBN 0-521-77752-6
  • Bonnie Blackburn and Leofranc Holford-Strevens. The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-reckoning. Oxford University Press; USA, 2000. pp 723-730.
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