Passover: Jewish holiday celebrating the Israelites' liberation from slavery in Egypt

They celebrate it to remember when God used Moses to free the Israelites from slavery in Egypt, as told in the book of Exodus in the Bible. God told Moses to set aside this special week originally called "the feast of unleavened bread". During this time, the people eat special foods, do special rituals and sing songs. Passover starts on the 15th of Nisan on the Hebrew calendar, which falls in March or April.

Passover
Official name	Hebrew: פסח (Pesach)
Observed by	Jews and Samaritans
Type	Jewish
Significance	Celebrates the Exodus, the freedom from slavery of the Children of Israel from ancient Egypt that followed the Ten Plagues. Beginning of the 49 days of Counting of the Omer
Begins	15th day of Nisan
Ends	21st day of Nisan in Israel, and among some liberal Diaspora Jews; 22nd day of Nisan outside of Israel among more traditional Diaspora Jews.
Celebrations	In Jewish practice, one or two festive Seder meals – first two nights; in the times of the Temple in Jerusalem, the Passover sacrifice. In Samaritan practice, men gather for a religious ceremony on mount Gerizim that includes the ancient lamb sacrifice.
Related to	Shavuot ("Festival of Weeks") which follows 49 days from the second night of Passover.

The "Second Passover" (Pesach Sheni) on the 15th of Iyar in the Hebrew Calendar is mentioned in the Hebrew Bible as a make-up day for people who were unable to offer the Passover sacrifice at the appropriate time.