If you have Jewish relatives or friends, there's a good chance you will be invited to join their celebration of Passover at a Seder. If so, it would be useful to know something about the historical background and unique practices of a rite that will be practiced in varying ways by Jews of all levels of observance on the first evening of Passover.

You may already know that the Seder is a commemoration of the Biblically narrated Exodus from Egypt. The Seder includes a retelling of that story, at least a sampling of the unleavened bread (matzoh) and bitter herbs featured in it, and lots of food and wine. Depending on the robustness of the family traditions, there can be lots of singing, too.

Typically, the celebrants at the Seder, which means order, will follow a sequence in a book called a Haggadah, which means telling. Depending on their familiarity with, and commitment to, ancient tradition, they will say blessings over four cups of wine, eat matzoh and bitter herbs, dip vegetables in salt water, hear four programmed questions about the seder from youngsters present, answer the questions with the story of Passover, chant hymns, Psalms, and songs, and invariably eat a lavish meal. The music can range from serious liturgy to folk songs and children's ditties.

Hundreds of editions of the Haggadah have been published in the United States, most with English translations and commentaries, so hitherto uninitiated guests have ample opportunity to follow the sequence and read enlightening explanations. Some recent editions have analogized the Jewish festival of freedom from slavery to more contemporary struggles for religious, racial, ethnic and gender rights.

For Christians, the wine and unleavened bread have a powerful association with the Last Supper and the central sacrament of their faith. All of the Gospels except John seem to place that meal on the first night of Passover, making it a Seder for the Master and his Jewish apostles. Whatever one's view of the body and blood concept, it had its antecedents in the sacrifice of the Passover lamb.

Ironically, that sacrifice is the one major element of the historical Passover not reenacted at contemporary Seders. On the eve of the departure from Egyptian bondage, the Israelites were ordered to sacrifice a lamb, place some of its blood on their doorposts, and ride out the night as a deadly plague struck the homes of their Egyptian neighbors. The miraculous events of that night are discussed in detail at the Seder but, since Jewish sacrificial rites can only be performed at the long-destroyed Temple in Jerusalem, the lamb is merely symbolized on the Seder table and in one of the unleavened wafers consumed at the end of the meal.

If you attend a Seder and like it, you'll be interested to know that many traditional Jews have a second one the next night. There's never too much of a good thing, some say.