Passover is a Jewish holiday that celebrates the liberation of Jews from slavery in ancient Egypt that took place at about 1300 BCE. Passover falls during the Hebrew month of Nisan which takes place during the Spring, and lasts for eight days in the diaspora (only seven in Israel). Jews traditionally commemorate Passover by participating in a Passover Seder or festival meal during the first two nights of Passover (only one night in Israel), and it is considered one of the most widely celebrated Jewish holidays.
During the Seder, it is tradition to tell the story of Passover and God calling on Moses to help free the slaves from Pharaoh. Pharaoh refused to listen and God inflicted ten plagues upon the ancient Egyptians before he would release the Jewish slaves. After the tenth and final plague, Pharaoh finally agreed to release the slaves and the Jewish people, led by Moses, fled Egypt in such a hurry that they could not wait for their bread dough to rise (leaven). This is why we celebrate Passover by eating unleavened bread or Matzah.