Holidays

Why 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 to keep holidays from 'wandering' into another season, adjustments must be made on leap years by adding an extra month of Adar.