Passover (Hebrew Pesach) commemorates the story of the Exodus, in which the ancient Israelites were freed from slavery in Egypt. Beginning on the 15th day of Nisan in the Jewish calendar, it is celebrated for seven or eight days. It is one of the most widely observed Jewish holidays.
In the narrative of the Exodus, the Bible tells that God helped the Children of Israel escape slavery in Egypt by inflicting ten plagues upon the Egyptians before the Pharaoh would release his Israelite slaves; the tenth and worst of the plagues was the death of the Egyptian first-born. The Israelites were instructed to mark the doorposts of their homes with the blood of a spring lamb and, upon seeing this, the spirit of the Lord knew to pass over the first-born in these homes, hence the name of the holiday. When the Pharaoh freed the Israelites, it is said that they left in such a hurry that they could not wait for bread dough to rise (leaven). In commemoration, for the duration of Passover no leavened bread is eaten, for which reason it is called "The Festival of the Unleavened Bread". Matzo (flat unleavened bread) is a symbol of the holiday.
The commandment to keep Passover is recorded in Leviticus:
In the first month, on the fourteenth day of the month between the two evenings is the Lord's Passover. And on the fifteenth day of the same month is the feast of unleavened bread unto the Lord; seven days ye shall eat unleavened bread. In the first day ye shall have a holy convocation; ye shall do no manner of servile work. And ye shall bring an offering made by fire unto the Lord seven days; in the seventh day is a holy convocation; ye shall do no manner of servile work. (Leviticus 23:5)
It is traditional for families or communities to gather on the first night for a Pesach Seder. The Haggadah is the text used for the Seder. Reading the Haggadah at the Seder table is a fulfillment of the Scriptural commandment to each Jew to "tell your son" of the Jewish liberation from slavery in Egypt as described in the Torah.
And thou shalt tell thy son in that day, saying: It is because of that which the LORD did for me when I came forth out of Egypt. (Exodus 13:8)
Together with Shavuot and Sukkot , Passover is one of the three pilgrimage festivals (Shalosh R'galim) during which the entire Jewish populace historically made a pilgrimage to the Temple in Jerusalem.