Passover is a celebration of the emancipation of the Israelites from being enslaved in ancient Egypt. It is an eight-day celebration from the 15th to the 22nd of the Hebrew month Nissan.
The Jewish calendar is based on lunar cycles of 29 1/2 days. Nissan falls approximately between March and April each year. Hebrew months are sometimes 29 days long and sometimes 30 days long, since they need to consist of complete days. In 2015, Passover starts the evening of Friday, April 3 and ends in the evening of Saturday, April 11. The highlight of the Passover celebration is the Seder meal, celebrated on the first two nights of Passover.