Though Jewish holidays are celebrated each year, the time of celebration depends on the lunar-based Hebrew Calendar, with each month beginning with the new moon. However, since Jewish holidays are also celebrating the solar seasons, Jewish leap years occur every two or three years.
Unlike the Gregorian calendar used by most of the world, which arbitrarily sets the length of months to 28, 30 or 31 days, so it corresponds to the 365.5 days of the Solar year, the Jewish calendar coordinates the astronomical characteristics of our cosmological environment, the months are either 29 or 30 days corresponding with the 29½-day lunar cycle and the years are either 12 or 13 months, corresponding to the 12.4 month solar cycle.
As a result, some years we celebrate Chanukah around Thanksgiving time and other years closer to New Years Day, and the high holidays are upon us at times in early to mid September and other years Rosh Hashanah, Yom Kippur and Sukkot/Simchat Torah are celebrated in October. You can read more about the Jewish holidays at http://www.Mazornet.com/holidays
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