The Hebrew calendar is the official calendar of Israel and is used for religious observances by Jews everywhere. The current Hebrew calendar is only a few centuries older than the Gregorian, though it incorporates many ancient elements. The names of the months are mostly Babylonian; they were all established by the 1st century ce. It was established by Hillel II in am 4119 (359 ce). Its complexity is almost overwhelming; it seems a full-employment program for those responsible for ascertaining when significant religious events should occur, which is the calendar's primary purpose. Time is calculated from the meridian of Jerusalem.
The era is that of the biblical Creation, which is placed in 3761 bce. Since the 9th century ce the years in this calendar have been styled A.M., for anno mundi (lizira or libri' ath `olam). In the Julian proleptic calendar, the year am 1 began on -3760 October 7.
Days begin at sunset, but for purposes of the calendar at 6 pm. Hours are divided into 1,080 halakim (sometimes romanized helaqim; singular, helek). One helek = 3 1⁄3 seconds. Each helek is divided into 76 rega`im (approximately 0.044 seconds).
The week contains seven days, six of which have names based on numbers, the seventh being the Sabbath.
Most years consist of 12 lunar months of either 30 days (male, a full month) or 29 days (haser, a defective month). Two of the months vary in length from year to year. To compensate for the 11-day difference between 12 lunar months and one solar year, an additional 30-day month is inserted in the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years of a 19-year cycle (derived from the 19-year lunar cycle). These years are 13-month leap years. Consequently the number of days in a year varies from year to year.
| Month | Number of days in the month | |||||
|---|---|---|---|---|---|---|
| common years | leap years | |||||
| complete (shelema) |
normal (sedura) |
defective (hasera) |
complete (shelema) |
normal (sedura) |
defective (hasera) |
|
| Tishri | 30 | 30 | 30 | 30 | 30 | 30 |
| Heshvan | 30 | 30 | 29 | 30 | 30 | 29 |
| Kislev | 30 | 29 | 29 | 30 | 29 | 29 |
| Tebet | 29 | 29 | 29 | 29 | 29 | 29 |
| Shebet | 30 | 30 | 30 | 30 | 30 | 30 |
| Adar* | 29 | 29 | 29 | 30 | 30 | 30 |
| II Adar or Veadar | — | — | — | 29 | 29 | 29 |
| Nisan | 30 | 30 | 30 | 30 | 30 | 30 |
| Iyyar | 29 | 29 | 29 | 29 | 29 | 29 |
| Sivan | 30 | 30 | 30 | 30 | 30 | 30 |
| Tammuz | 29 | 29 | 29 | 29 | 29 | 29 |
| Av | 30 | 30 | 30 | 30 | 30 | 30 |
| Elul | 29 | 29 | 29 | 29 | 29 | 29 |
| days in year | 355 | 354 | 353 | 385 | 384 | 383 |
*Called I Adar in leap years.
By altering the pattern of years within the 19-year cycle, it is possible to prevent holidays from falling on days that would be religiously unacceptable. For example, the Day of Atonement (Yom Kippur, Tishri 10) must not fall on a Friday or Sunday, so New Year's (Rosh Hashana, Tishri 1) must not fall on a Wednesday or Friday. These adjustments are made, however, in such a fashion that it all comes out in the wash at the end of every 19-year cycle, and over the long run the seasons remain in place.
J. R. Cohen.
Mishnah Tractate Rosh Hashanah..
New York, 1981.
E. Mahler.
Handbuch der jüdisch Chronologie.
1916.
L. A. Resnikoff.
Jewish calendar calculations.
Scripta Mathematica, volume 9, pages 191–195, 274–277 (1943).
A. Spier.
The Comprehensive Hebrew Calendar.
New York, 1952.
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Last revised: 4 October 2005.