Passover, the Jewish festival in celebration of the Jews' freedom from slavery and the flight from Egypt, begins this year at sunset on April 18.

Although traditions vary throughout the world, the basics are as follows: The holiday lasts seven or eight days (depending on where it's being celebrated), and the first night of Passover begins with a ceremonial dinner, called a Seder, where the story of the Exodus is told.

The food and wine customs of a given Seder are elaborate, and differ between regions and families, but some factors remain constant.

Each participant in the Seder drinks four cups of wine throughout the evening, at fixed points, for the four promises of redemption associated with the Exodus story.

The major dietary restriction during the week of Passover is the ban of leavened bread, or chometz. Chometz is bread made from (wheat, oat, spelt, rye or barley) flour that has been in contact with water for more than 18 minutes and therefore had a chance to rise.

Before Passover, the house is traditionally cleansed of chometz.

Fundamental to the Seder table is the Seder plate, which has on it the following items:

Ÿ zeroah, a lamb's shank bone symbolizing the ancient Passover sacrifice;

Ÿ beitzah, a roasted egg symbolizing the temple sacrifice and the continuing cycle of life;

Ÿ haroset, a paste of fruit and nuts symbolizing the mortar used to build the pyramid of the pharaohs;

Ÿ mar'or, a bitter herb (like horseradish) to represent the bitterness of slavery;

Ÿ karpas, a green vegetable (usually parsley) representing spring;

Ÿ a bowl of salt water to dip the karpas symbolizing the slaves' tears

Some traditions also include chazeret, a second bitter herb, usually the roots of romaine lettuce. Also necessary are three matzos (unleavened bread, symbolizing the haste of the flight from Egypt there was no time for the bread to rise), either wrapped in cloth or covered, and broken and eaten at set points throughout the evening.

The actual Seder meal is also quite variable. Traditions among Ashkenazi Jews generally include gefilte fish (poached fish dumplings), matzo-ball soup, brisket or roast chicken, potato kugel (somewhat like a casserole) and tzimmes, a stew of carrots and prunes, sometimes including potatoes or sweet potatoes.