The Jewish Calendar
People often say: "The Jewish holidays are late this year" or "The Jewish 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 29.5 days to complete its cycle; 12 lunar months equal 354 days. A solar year is 365 1/4 days. There is a difference of 11 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 19 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.