The first six months of the Jewish calendar are Nissan, Iyar, Sivan, Tammuz, Av and Elul. The final six months of the Jewish calendar are Tishri, Cheshvan, Kislev, Tevet, Shevat and Adar. On leap years, a thirteenth month is added to the end of the Jewish year.
While modern calendars, such as the Gregorian calendar used by Western society, are based on the solar cycle, the Jewish calendar uses both the lunar and solar cycles. Months in the Jewish calendar are 29 or 30 days, corresponding with the length of a lunar cycle. However, a Jewish calendar has 12 or 13 months, following the solar rule that other calendars follow.
A Jewish month begins when the moon is first visible following the day of the new moon phase. Because a lunar cycle takes approximately 29 1/2 days, some months have 30 days to compensate. Similarly, because the solar cycle is approximately 12.4 months, the Jewish calendar provides for seven leap years every 19 Jewish years. On a leap year, an extra month of 29 days is added to the end of the calendar; the month is usually referred to as Adar II.
Because one of the most important events in the Jewish religion is Passover, the first month of the year begins around that event. Therefore, the ecclesiastical Jewish new year falls in either March or April in the Gregorian calendar. However, the civil Jewish New Year begins in Tishri, which usually falls around September or October.