Unleavened bread is bread that is made without yeast or other leavening. This kind of bread is flat because it has no leaven to make it rise.
When God freed the Israelites from slavery in Egypt, he told them to flee Egypt quickly without waiting for their bread to rise. So they ate unleavened bread with their meal. After that, the Israelites ate unleavened bread each year during the Festival of Unleavened Bread. They did this to remember when God saved them from Egypt.
This festival is celebrated in the first month of the Hebrew calendar. Passover begins on the fourteenth day of the first month when the sun goes down. The Feast of Unleavened Bread starts on the fifteenth day as Passover ends. The festival lasts for seven days (see: Exodus 12:14-20; Leviticus 23:4-8; Deuteronomy 16:1-8; see also: Matthew 26:17; Mark 14:1; Luke 22:1, 7; Acts 12:3; 20:6).
The Festival of Unleavened Bread is one of three festivals for which God commanded all male Israelites to travel to Jerusalem and bring him an offering. The other two festivals are the Festival of Harvest or Pentecost and the Festival of Shelters (see: Exodus 23:14-17).