The name Pesach is derived from the Hebrew word pasach, which means "passed over," which is also the source of the common English name for the holiday. It recalls the miraculous tenth plague when all the Egyptian firstborn were killed, but the Israelites were spared.
The story of Passover originates in the Bible as the telling of the Exodus from Egypt. The Torah recounts how the Children of Israel were enslaved in Egypt by a Pharoah who feared them. After many generations of oppression, God speaks to an Israelite man named Moses and instructs him to go to Pharoah and let God's people go free. Pharoah refuses, and Moses, acting as God's messenger brings down a series of 10 plagues on Egypt.
The last plague was the Slaying of the Firstborn; God went through Egypt and killed each firstborn, but passed over the houses of the Israelites leaving their children unharmed. This plague was so terrible that Pharoah relented and let the Israelites leave.
Pharoah then regretted his decision and chased the Children of Israel until they were trapped at the Sea of Reeds. But God instructed Moses to stretch his staff over the Sea of Reeds and the waters parted, allowing the Children of Israel to walk through on dry land. The waters then closed, drowning Pharoah and his soldiers as they pursued the Israelites.
The Torah commands an observance of seven days of Passover. Many Jews in North America and all Jews in Israel follow this injunction. Some Jews outside of Israel celebrate Passover for eight days. The addition of a day dates back to 700-600 B.C.E. At that time, people were notified of a holiday’s beginning by means of an elaborate network of mountaintop bonfires. To guard against the possibility of error, an extra day was added to many of the holidays. Today, a dependable calendar exists, allowing Jews to know when holidays start and end. However, the process remains ingrained in Jewish law and practice for some Jews living outside of Israel today.