The months of the Hebrew calendar (Nisan, Iyar, Sivan, Tammuz, Av, Elul, Tishrei, Cheshvan, Kislev, Tevet, Shevat and Adar), belong to a lunisolar system in which each month begins at the new moon. To keep the festivals in their proper seasons, a thirteenth month, Adar II, is added in seven years of every nineteen, in the fixed Metonic cycle that governs the calendar.
Each month carries its festivals and observances: Nisan holds Passover, Tishrei the High Holy Days of Rosh Hashanah and Yom Kippur and the festival of Sukkot, Kislev the beginning of Hanukkah, Adar the feast of Purim.