Introduction to the Jewish Calendar
Days and Weeks
The Jewish day begins at sunset. The status of the period between sunset (the disappearance of the sun behind the horizon) and nightfall (the emergence of three medium-sized stars) is doubtful. For some purposes, it is treated as part of the previous day, e.g. at the end of Shabbat, when the prohibition of creative activities (melacha) remains in force until nightfall.
Books and computer programs for conversions between the Jewish and Gregorian (civil) calendars are based on the daylight portion of the Jewish day. For instance, if you know that one of your ancestors was born on 26 Nissan 5580, you will find that this corresponds to 10 April 1820 — but the actual birthday may have been the previous day, 9 April 1820, in the evening.
With the exception of the Shabbat, the weekdays have no names. They are simply numbered:
The week culminates in the seventh day, the Holy Shabbat (shabbat kodesh, abbreviated ).
The Jewish month is based on the lunar or synodic month, the time it takes for the moon to circle the earth. Since the exact duration of one revolution is a little over 29.5 days, the length of the months normally alternates between 29 and 30 days. A month of 30 days is called male ('full'), one of 29 days is chaser ('defective'). There are two months which are male in some years and chaser in others.
The month begins with the appearance of the new moon. In the time of the Temple, the Sanhedrin (the highest court) sanctified the new month when two witnesses had actually sighted the moon. In the middle of the fourth century C.E., a fixed calendar was introduced.
In the Torah, the months are numbered; the first is the one in which the Exodus from Egypt occurred (Yetziat Mitzrayim; cf. Shemot [Exodus] 12:2). Later, names of Babylonian origin were adopted:
The first day of each month (with the exception of Rosh Hashana, the Jewish New Year) is Rosh Chodesh (lit. 'head of the month', abbreviated ) — and so is the thirtieth day of the preceding month, if there is one. For example, if a gravestone inscription mentions the first day of Rosh Chodesh Elul, the calendar date "30 Av" is meant.
An ordinary year consists of twelve months. When Cheshvan has 29 days and Kislev 30, it is "regular" (kesidra); if both have 30 days, it is "complete" (sh'lema) or "excessive"; and if both have 29 days it is "defective" (chasera). Thus, an ordinary year can have 353, 354 or 355 days.
A lunar year of 354 days is about 11 days shorter than the solar year, i.e. one revolution of the earth around the sun, which corresponds to the cycle of the seasons. If the Jewish calendar were based exclusively on the lunar year, Pesach (15 Nisan) would fall in the spring in one year, in the winter a few years later, then in the autumn, then in the summer and – after about 33 years – in the spring again. But the Torah says that Pesach must be celebrated in the spring (be-chodesh ha-aviv, Shemot [Exodus] 13:4), and so the average length of the Jewish year must be adjusted to the solar year. This is achieved by adding an entire month about every three years: In each cycle of 19 years, the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years are leap years, the others are common years. For example, 5755 was a leap year because it was the 17th year in the 303rd cycle of 19 years: 5755/19 = 302 + 17/19. (This is something that you can calculate online.)
The extra month in a leap year has 30 days, so that the year lasts for 383, 384 or 385 days. It is added after the month of Sh'vat and is called Adar I, whereas the original Adar (of 29 days) becomes Adar II. Purim, which is on 14 Adar, is celebrated in Adar II in a leap year. Someone who was born in Adar of a common year will celebrate the anniversary in Adar II in leap years, but yahrzeit for someone who died in Adar of a common year is observed in Adar I in leap years.
The new year begins with Rosh Hashana, the first of Tishri (although this is the seventh month), in September or early October according to the Gregorian (civil) calendar. Jewish years are counted from the Creation of the world. To convert the Jewish year to the year of the Common Era (CE), subtract 3760 (or 3761 for the first months; in most years, 1 January falls in Tevet). For example, the major part of the Jewish year 5678 corresponded to 1918; the beginning of 5678 was in 1917. When the year is written with Hebrew letters, the 5000 is usually omitted ("small count", abbreviated ). In that case, one can find the civil equivalent by adding 1240. For instance, the numerical values of the letters add up to 756, short for 5756. That is the Jewish year which corresponds to 1996 (756 + 1240 = 1996); to be precise, it lasts from the evening of 24 September 1995 until the evening of 13 September 1996. (The numerical equivalent of a year written in Hebrew letters can be determined online.)
All Jewish holidays, fast days, remembrance days etc. have a fixed date in the Jewish calendar. Some of them are shifted to a different day if they fall on or just before the Shabbat.
There are numerous calendar conversion programs available, including:
Copyright ©1996 Joachim Mugdan, All rights reserved.
Created 3 Feb 1996. Revised 15 Dec 1997 / 19 Apr 1999 / 3 Apr 2003 / 15 Nov 2006 / 20 Oct 2011 WSB