The Jewish calendar originated during the Babylonian exile in the 6th century BC. The months are calculated according to the moon and the year according to the sun. The Julian-Gregorian calendar used around the world is based on the duration of one orbit of the earth around the sun. Its name derives from Julius Caesar, who introduced this system, and from Pope Gregory XIII, who had it finalized around 1580. With 365 days, the calendar year does not correspond exactly to the solar year, necessitating the insertion of an additional day every four years. A lunar year, on the contrary, is based on the basic unit of time from one new moon to the next. The lunar year is eleven days shorter, so that fixed points such as public holidays fall in different seasons. Ramadan, the Muslim month of fasting, can fall just as easily in summer as in winter.
Because the Jewish Pesach festival has to fall in the spring due to the stipulations of the Torah, the two systems are brought together in the Jewish calendar with the insertion of eleven days. In a 19-year cycle, an additional month called Adar II is added after the month of Adar. Such a year is called a “pregnant” year. Adar is the sixth month in the Jewish calendar and falls in February or March.