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The Jewish Calendar

A Brief Introduction to the Jewish Calendar:

The Jewish calendar that dates from the time of Hillel II (359 C.E., A.M. 4119) is an official calendar of the State of Israel, along with the Gregorian calendar. It is a lunisolar calendar based on computations rather than visual observations; sightings of the young crescent moon were used in ancient times. The initial epoch for the new Moon is 11:11:20 p.m. at the meridian of Jerusalem on Monday, 7 October 3761 B.C.E. (proleptic Julian calendar; Jerusalem's longitude is 2 hr 21 min east of Greenwich). This began the Jewish calendar, 1 Tishri A.M. 1 (Anno Mundi).

The length of the synodic month used in the Jewish calendar is 29 days 12 hours 44 minutes 3⅓ seconds. The years in the Jewish calendar are counted in cycles of 19 years; of which, 12 years are common years of 12 months apiece, and 7 years are leap years containing 13 months. Altogether, the 19 year cycle has 235 months. Leap years are now fixed as the 3rd, 6th, 8th, 11th, 14th, 17th and 19th year of each cycle.

The names of the 12 months of the Jewish calendar, transliterated into the Roman alphabet, and their formal tabular lengths are:

Name Length
Tishri 30
Heshvan 29 a
Kislev 30 b
Tebet 29
Shebat 30
Addar 29 c
Nisan 30
Iyar 29
Sivan 30
Tammuz 29
Ab 30
Elul 29

a In an abundant year, Heshvan has 30 days.
b In a deficient year, Kislev has 29 days.
c In a leap year, Addar has 30 days; it is followed by Addar II with 29 days.


Rules for Extending the Year

(Deficient, Regular, and Abundant Length Years)

The Jewish calendar year begins with the first day of Rosh Hashanah (1 Tishri). This date is determined by four rules which can postpone 1 Tishri by one or two days after the fictitious new moon. The calculated period between new moons in the Jewish calendar is 29.530594 days, which on average remains nearly in sync with the actual new moon (The synodic month is slowly increasing; at present the it is about 0.46s longer than the Hillel II value.), but does not coincide with the actual time of new moon.

  1. If the fictitious new moon falls at or after noon, 1 Tishri is postponed by one day. If this causes 1 Tishri to fall on Sunday, Wednesday, or Friday, then 1 Tishri is postponed an additional day to satisfy Rule 4.
  2. If the fictitious new moon of a common year falls on a Tuesday at or after 3:11:20 a.m., then 1 Tishri is postponed one day to Wednesday. Rule 4 would then require additional postponement to Thursday.
  3. If the new moon of a year following a leap year falls on a Monday, at or after 9:32:43⅓ a.m., then 1 Tishri is postponed one day to Tuesday.
  4. If the new moon falls on a Sunday, Wednesday, or Friday, then 1 Tishri is postponed by one day.

The first three rules are considered astronomical rules, while the fourth is a political rule. Only one of the astronomical rules applies in any year, but both an astronomical rule and the political rule can be applied in the same year.

If none of these rules apply, it is a deficient year. If one of the rules applies, it is a regular year. If two rules apply, it is an abundant year.

Days in Hebrew Year
Common YearLeap Year
Deficient353383
Regular354384
Abundant355385

Observances in the Jewish Calendar

The dates for several observances in the Jewish calendar for the years 360 C.E. or later using the civil calendar (Julian or Gregorian) are available here.

For further information on calendars, see Richards, E.G. 2012, "Calendars," from the Explanatory Supplement to the Astronomical Almanac, 3rd edition, S.E Urban and P.K. Seidelmann eds., (Mill Valley, CA: University Science Books), Chapter 15, pp. 585-624.