The Jewish calendar combines both lunar and solar cycles. The calendar is of no fixed length and has different starting points for different purposes. Jews use the calendar to determine holiday dates and appropriate timing for public reading Torah portions and daily psalms.
The appearance of a thin crescent moon signals the beginning of a new Jewish month. Each month takes 29 or 30 days. The first day of each month is the Roch Chodesh which means the "head of the month." The Jewish day begins and ends at sunset thus the Sabbath starts on Friday at sundown and not on the midnight of Saturday.
Nissan is the first month of the Jewish calendar. It starts from March to April in the Gregorian calendar and it is a special month where the Jews mark the Passover feast. Iyar, Sivan and Tammuz are the second, third and fourth months respectively spanning from mid-April to July in the Gregorian calendar.
Menachem Av is from mid-July to August while Elul, Tishrei, Marcheshvan and Kislev represent the fifth to the ninth months respectively. Tevet starts from mid-December to January. Shevat and Adar are the last two months. In a leap year, an additional month Adar I is added after the eleventh month and the regular Adar becomes Adar II.