The Jewish Calendar
Jews often say: "The holidays are late this year" or "The holidays are early this year." In fact, the holidays never are early or late; they are always on time, according to the Jewish calendar.
Unlike the Gregorian (civil) calendar, which is based on the sun (solar), the Jewish calendar is based primarily on the moon (lunar), with periodic adjustments made to account for the differences between the solar and lunar cycles. Therefore, the Jewish calendar might be described as both solar and lunar. The moon takes an average of twenty-nine and one-half days to complete its cycle; twelve lunar months equal 354 days. A solar year is 365 1/4 days. There is a difference of eleven days per year. To ensure that the Jewish holidays always fall in the proper season, an extra month is added to the Hebrew calendar seven times out of every nineteen years. If this were not done, the fall harvest festival of Sukkot, for instance, would sometimes be celebrated in the summer, or the spring holiday of Passover would sometimes occur in the winter.