The Passover Seder is a ritual feast at the beginning of the Jewish holiday of Passover. It is conducted throughout the world at the start of the 15th day of Nisan in the Hebrew calendar (at sunset, when a Hebrew day begins). The day falls in late March or in April of the Gregorian calendar. Passover lasts for seven days in Israel, and customarily usually eight days in the Jewish diaspora. A seder is held on the first night, the 15th of Nisan; where eight days are observed, a seder is often also held on the second night.
The Seder is a ritual involving a retelling of the story of the liberation of the Israelites from slavery in ancient Egypt, taken from the Book of Exodus (Shemot) in the Torah. The Seder itself is based on the Biblical verse commanding Jews to retell the story of the Exodus from Egypt: "You shall tell your child on that day, saying, 'It is because of what the LORD did for me when I came out of Egypt.'" (Exodus 13:8) At the seder, Jews read the text of the Haggadah, an ancient Tannaitic work narrating the Israelite exodus from Egypt, with special blessings and rituals, Talmudic commentaries, and Passover songs.