The Hebrew calendar is a lunisolar calendar used primarily for Jewish religious observances. It consists of 12 months in a common year and 13 months in a leap year, which occurs seven times in a 19-year cycle. Each month begins with the sighting of the new moon, making it different from the Gregorian calendar, which is purely solar. The months of the Hebrew calendar include Nisan, Iyar, and Elul, among others. Important Jewish holidays, such as Passover and Yom Kippur, are based on this calendar. The year is counted from the creation of the world, which is traditionally dated to 3761 BCE.