A variety of calendars was already in use in Judaism in biblical times. An earlier lunar calendar was replaced by a lunisolar calendar of Babylonian origin, based on astronomical observations, which became increasingly exact in the postbiblical period. This calendar was in use both in the Land of Israel and in the entire Diaspora, where competing non-Jewish-majority calendars were also in use, to ensure unified and exact observation of religious festivals (Course of the Year).…