The Hebrew calendar is quite different from the Gregorian calendar, which is commonly used in the modern world. Unlike the Solar year, which consists of approximately 365.25 days, the Lunar Year, which is used in the Hebrew calendar, only has 354 days. Each cycle of the moon around the sun lasts 29.5 days (to be exact- 29 days, 12 hours, 44 minutes and 3.33 seconds?), and therefore some Hebrew months have 29 days while others have 30 days. However, the Hebrew calendar is not at all unrelated to the Solar year: There is a lot of importance to the four seasons in the Jewish religion, and so once every 3 years an extra month is added to the Hebrew calendar in order for it to synchronize with the solar calendar. From that point of view, the Hebrew calendar is both lunar and solar!
According to Jewish tradition, each day begins at sunset and ends at sunset. That is how Sabbath is observed, and that is also how most Jewish holidays are celebrated.
But when does a month begin? "Rosh Chodesh", the beginning of a Jewish month, occurs when the dark phase of the moon is over and a thin silvery moon appears. In ancient times the beginning of a month was announced when two unrelated people stated that they had seen the moon. Nowadays the beginning of each month of the year is known in advance, thanks to careful calculations that were introduced by Hillel, in the year 359 C.E. Once every 19 years the solar year and the Hebrew year begin on the same day, and a new cycle begins. Once every 247 years the Hebrew and Gregorian calendars merge together with the day of the week. But even then, the Hebrew year is 50 minutes shorter than the Solar year, and this causes a very slow shift "backwards": Since this method of counting was introduced, the time of the Solar year in which Passover is celebrated moved about one week back, in the direction of winter.
The concept of adding a 13th month once every three years is called "Shana Meuberet" (literally, "a pregnant year"), or a Leap Year. The extra month which is added is another "Adar", and on such a year we have "Adar Aleph" and "Adar Bet". There are two reasons for doubling Adar (rather than any other month):
Tishrei, formally the new year started at the beginning of Nisan. Therefore, it seemed only natural to place an additional month at the end of the year.
However, the additional month is in fact not the later one, but the earlier one. In a leap year Purim is celebrated in Adar B, and so are birthdays, Bar Mitzvah ceremonies, and other happy occasions. The 7th day of Adar, which is the memorial for Moses, is observed in Adar B as well. On the other hand, all other memorials are held during the first Adar.
Nowadays, thanks to precise calculations, the date of the New Year is known well in advance. But it wasn't always like that?
Until 359 C.E. many years began a day later than they were supposed to. There were several conditions in which the beginning of the New Year was postponed:
Consequently, although a normal year has 354 days, it is possible for a year to be a day longer (thus called a "full year") or a day shorter (thus called a "missing" year).
|Name of month||Number of days|
|Cheshvan||29 (30 on a "full" year)|
|Kislev||30 (29 on a "missing" year)|
|Adar||29 (Adar Aleph- 30, Adar Bet- 29)|