The months of the Jewish or Hebrew calendar begin with Nissan, Iyar, Sivan, Tammuz, Av and Elul. The calendar continues with Tishri, Cheshvan, Kislev, Tevet, Shevat, Adar (called Adar I in leap years only) and Adar II, which is called Adar Beit during leap years.
The names of the Jewish months date back to the biblical time of Ezra and the period directly following the end of Jewish captivity in Babylon. Linguistically, the names of the months were imported into Hebrew from Babylonian. Most of the months are either 29 or 30 days in length, with the lead month, Nissan, beginning in the civil equivalent of March/ April. Passover is celebrated in Nissan.
Because of the different dating system and extensive history of the Jewish people, years in the Hebrew calendar are also calculated differently from the common calendar, with the Jewish New Year commencing in the seventh month, Tishri. The exact lengths of the months of Cheshvan and Kislev are also dependent on Tishri.
Whereas the civil calendar is a solar one, the Jewish calendar operates as a "luni-solar" model, and is thus usually only 354 days as opposed to 365. Accordingly, the Jewish calendar adds an extra month (Adar II) every two or three years to compensate for the resulting 11-day lag.