The Jewish calendar is a lunisolar calendar used primarily for religious observances in Judaism. It consists of 12 months in a common year and 13 months in a leap year, aligning with both the lunar cycle and the solar year. Each month begins with the new moon, making it about 29 or 30 days long. Key months include Nisan, which marks the beginning of the year and the celebration of Passover, and Tishrei, which includes important holidays like Rosh Hashanah and Yom Kippur. The calendar also features various festivals and fast days, reflecting the agricultural and historical significance of the Jewish tradition.