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How the Jewish Calendar Works
(Or Why is Rosh Hashanah so late this year?)

Holiday Chart  (see below)

The Jewish Year

The Jewish holidays always fall on their proper dates. Chanukah always begins on the 25th of Kislev; Rosh Hashanah always falls on the 1st of Tishre. Why then does Rosh Hashanah sometimes fall at the beginning of September, and sometimes fall as late as the beginning of October? The Jewish and Julian calendars are not identical.

The Torah's word for month (chodesh) comes from the root 'chadash' (meaning new) because each Jewish month begins with a new moon. Even in English, we can recognize that the word month is related to the word for moon, but the English month can be 28, 29, 30 or 31 days long. The Jewish month follows the lunar cycle and is 29 (or 30) days long. (Note also: in English, the 'new moon' actually is 'no moon'; in our context, the new moon is when the moon is first sighted.)

Unlike the Julian or secular calendar, where months are no longer bound to their lunar origins, in the Jewish calendar, a new (Hebrew) month always coincides with the new moon, and the full moon always falls on the fifteenth of the Hebrew month. (Note that many Jewish holidays and special dates- Sukkot, Tu B'Shevat, Purim, Passover, Tu B'Av- occur in the middle of the month: on the night with the most available natural light!) Compare this to the Julian calendar where the new moon and full moon have to be graphically indicated on calendars, as they can move around from the beginning to the end of the month. There can even be (rare but possible) an English month with two full moons, (the second one called a 'blue moon,' heard in the expression 'once in a blue moon' to mean very rarely).

Measuring the months so faithfully according to the lunar cycle creates one small problem. Since many of the Jewish holidays are season-specific (Passover is a holiday of spring) it cannot move more than a few weeks either way, and so the 'lunar' months must also be calibrated to the [solar] seasons. The present Jewish calendar is therefore 'lunisolar,' and adjustments must be made to keep holidays from 'wandering.'

Here is the math: The lunar cycle (the time it takes to go from one new moon to another) is twenty nine and a half (24 hour) days. Because of the difficulty it would create trying to live with a half day in the calendar, Jewish months alternate between 29 and 30 days. (Otherwise, every other month we would have to alternate between working a daytime and nighttime shift!) Some of you are already pulling out your calculators and figuring out a discrepancy: if we add up six 29 day-months and six 30-day months (go ahead- I'll wait) we get 174+180=354-- eleven days short of the 365.25 days of a solar year. (Some lunar calendars live with this.The Moslem calendar is a wholly lunar year of 354 days. This means that holidays (and birthdays) will slowly migrate across the seasons. The month of Ramadan can occur in any season. Moslem children born in the summer celebrate their 18th birthday in the middle of winter!)

After three years, the eleven day discrepancy adds up to 33 days- enough for a whole month! So the solution is to add a leap month (of Adar) once every three years (approximately) to compensate for the difference; (Actually 9 times in a 17 year cycle). This inserted month is called Adar Beit (Adar II), and when there are two months of Adar, the holiday of Purim is celebrated in the second Adar.

This extra month also changes the number of weeks in a year. Hence, the 54 Torah portions read each week sometimes need to be 'doubled up' in a regular year. These double portions are called 'parashiyot mechubarot.' See "Why do we sometimes read two parashiyot on Shabbat?"

*Jewish holidays begin at sundown on the day before the listed date.