What historical event is celebrated by the passover?

The Passover is a Jewish festival that commemorates the the first Passover that occurred (according to the Bible) during the reign of Ramases the Great in Egypt. The word passover is used to describe the moment in which the angel of death descended upon Egypt and proceeded to kill every firstborn Egyptian child, however the Israelite slaves living in Egypt were spared this tragedy as they were favored by God thus the angel of death 'passed over' their homes. This event in turn allowed the Israelites to be freed from slavery. This event is now commemorated every year in Jewish families to remember that God delivered their people out of slavery and that he will never abandon them. If you want more context then feel free to read the book of Exodus which accounts all the happenigs in great detail.