(From Wikipedia) Passover, also called Pesach or Pesakh (/ˈpɛsɑːx, ˈpeɪ-/; Hebrew: פֶּסַח Pesaḥ) is a ritual meal, the Pesach seder, that occurs the night of the paschal full moon after the 14th of Nisan, eve of the 15th, telling the story of the exodus, and remembering how the angel of death “passed over” the houses of the Israelites during the tenth plague on Egypt. It begins Feast of Unleavened Bread that continues through the 22nd of Nisan, and is one of three pilgrimage festivals in which all Jewish males living in the land of Israel are obliged. On the 16th of Nisan, jews begin the counting of the omer, the memorial offering of the firstfruits of the barley harvest. The counting continues for seven weeks until the Feast of Shavuot, also known as Pentecost.
When the Temple in Jerusalem stood, the paschal lamb was offered and eaten on Passover eve, while the wave offering of barley was offered on the second day of the festival. Nowadays, in addition to the biblical prohibition of owning leavened foods for the duration of the holiday, the Passover seder is one of the most widely observed rituals in Judaism. In the Diaspora the unleavened bread is celebrated for 8 days, based on the concept of yom tov sheni shel galuyot.