Passover commemorates the Exodus, the historical deliverance of the Jewish people from Egyptian bondage in the days of Moses (13th century BC), Jews are ever mindful that this event was a prelude to God's revelation on Mount Sinai. 

A holiday commemorating the Hebrews' liberation from slavery in Egypt and the "passing over" of the forces of destruction, or the sparing of the firstborn of the Israelites, when the Lord "smote the land of Egypt" on the eve of the Exodus. The festival thus marks the first and most momentous event in Jewish history. Passover begins with the 15th and ends with the 22nd (or, outside of Israel and among Reform Jews, the 21st) day of the month of Nisan (March or April). On these seven (or eight) days, all leaven, whether in bread or other mixture, is prohibited, and only unleavened bread, called matzo, may be eaten. The matzo symbolizes both the Hebrews' suffering while in bondage and the haste with which they left Egypt in the course of the Exodus, not allowing time for the baking of  bread.   The dough was made without  allowing time for it to rise  and was later baked in the desert sun, and so we have matzo. 
 
 

Passover is also sometimes called the Festival of Unleavened Bread.