Celebrated for seven or eight days beginning on the 15th day of Nissan (the seventh month), Passover commemorates the Exodus from Egypt. It is also the ancient Hebrew New Year. On the first day or two days, Jews have a festival dinner called a seder, where they retell the story of the Exodus from a book called a haggadah. Jews are required to abstain from eating or owning leavened bread, and anything made with leaven, for the duration of the festival; matzah (a flat unleavened bread) is eaten instead. On Passover, the Song of Songs is recited. Passover also begins a period of seven weeks called the Omer, a period of semi-mourning that leads into Shavuot.