The Jewish calendar is lunisolar—that is, it is regulated by the positions of both the moon and the sun. There are twelve alternating lunar months of twenty-nine and thirty days each. The year totals 353, 354, or 355 days. Leap years are introduced to conform to the solar year of 365.24 days. Leap years contain either 383, 384, or 385 days and occur seven times in every nineteen-year period. This is called the Metonic cycle. The first month of the year is that in which the Exodus began. But tradition dictates that certain feasts must occur during certain seasons, so the calendar has to be adjusted every so often to prevent the lunar months from straying too far from the agricultural, or solar, cycle. To make it work, an extra month, called Adar Sheni (Second Adar), is added during seven out of every nineteen years. Thus, the number of days in a given year is not fixed and may vary from 353 to 385 days, and the first day of the month can fall on any day of the week and will vary from year to year. The Jewish lunar months are called Tishri (September/October), Cheshvan (October/November), Kislev (November/December), Tevet (December/January), Shevat (January/February), Adar (February/March), Adar Sheni (Second Adar, inserted only in “leap years”), Nisan (March/April), Iyyar (April/May), Sivan (May/June), Tammuz (June/July), Av (July/August), and Elul (August/September). With the leap year provision, the lunar months slide back or forward but remain within the solar months indicated in parentheses.