Passover is commemorated through a celebration that lasts seven or eight days. During the celebration, the story of the Israelites fleeing from Egypt is told and songs are sung.
Often, a Passover celebration starts by a thorough house cleaning to rid the home of any leaven. Tradition states that the Jews escaped Egypt in such a hurry that they didn't have time to let the bread rise, which occurs through leavening.
The Seder ceremony is traditionally celebrated on the first night of Passover, but many Jews also hold Seders on other nights during the holiday. The Seder starts out with a ceremony involving a lamb bone, a roasted egg, green vegetables dipped in salt water, bitter herbs and charoset (a type of an apple), walnut and wine concoction. The ceremony also includes matzo, which is an unleavened bread and the only type of bread allowed during Passover. Four cups of wine are placed at the table to symbolize the four expressions of freedom. An extra place is set and left empty for Elijah the prophet, who is expected to come to announce the coming of the Messiah.
After the meal, the tale of the Israelites' flight from Egypt is read from the Haggadah, a guide to the ceremony, and songs are sung in celebration.