Passover is a Jewish festival, celebrated on April 11, 2017. It commemorates the story of the Exodus, in which the ancient Israelites were freed from slavery in Egypt. The Passover begins on the 15th day of the month of Nisan, which typically falls in March or April of the Gregorian calendar. It is a spring festival, so the 14th day of Nisan begins on the night of a full moon after the northern vernal equinox. Passover is celebrated for seven or eight days. It is one of the most widely observed Jewish holidays.
Together with Shavuot ("Pentecost") and Sukkot ("Tabernacles"), Passover is one of the three pilgrimage festivals (Shalosh Regalim) during which the entire Jewish populace historically made a pilgrimage to the Temple in Jerusalem. Samaritans still make this pilgrimage to Mount Gerizim, but only men participate in public worship. It is traditional for Jewish families to gather on the first night of Passover (first two nights in communities outside the land of Israel) for a special dinner called a seder.
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-borns in these homes, hence the name of the holiday. (With material from: Wikipedia)