Final answer:
Passover is the Old Testament feast that celebrates the protection of Israelite firstborns during the tenth plague in Egypt, as recounted in the book of Exodus.
Step-by-step explanation:
The Old Testament feast that celebrated God's sparing of the firstborn of Israel the night they left Egypt is Passover. This feast commemorates the events described in the book of Exodus, where the Angel of Death 'passed over' the homes of the Israelites that were marked with a red symbol, protecting their children during the plague that took the lives of firstborns in Egypt. This act of salvation and the subsequent release of the Israelites from bondage are remembered each year during the Passover seder, a special ritual meal that involves retelling the story of the Exodus, including the miraculous escape from slavery and the eventual journey to Canaan.