Pesach starts on the 15th day of the Hebrew month of Nisan,

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which is considered the first month of the Hebrew year.

According to the Book of Exodus, God commanded Moses to tell the Israelites to mark a lamb's blood above their doors so that the Angel of Death would pass over them: they would not be touched by the tenth Plague of Egypt, the death of the firstborn. After this Plague, Pharaoh ordered the Israelites to leave, taking whatever they wanted, and asked Moses to bless him in the name of God. The passage goes on to state that the Passover sacrifice recalls the time when God "passed over the houses of the Israelites in Egypt".

https://en.wikipedia.org/wiki/Passover