The Short Jewish Passover Story:
Being a slave for many decades to the Egyptian pharaohs, during which time the Israelites were subjected to backbreaking labor and unbearable horrors, G‑d sent Moses to Pharaoh with a message: “Send forth My people, so that they may serve Me.” After and despite numerous warnings, Pharaoh refused to heed G‑d’s command. G‑d then sent ten devastating plagues upon Egypt , afflicting them and destroying everything from their their crops to their livestock.
At the stroke of midnight of 15 Nissan in the year 2448 from creation (1313 BCE), G‑d visited the last of the ten plagues on the Egyptians, killing all their firstborn. While doing so, G‑d spared the children of Israel, “passing over” their homes—hence the name of the holiday. Pharaoh’s resistance was broken, and he virtually chased his former slaves out of the land. The Israelites left in such a hurry, in fact, that the bread they baked as provisions for the way did not have time to rise. They left Egypt on that day and began the trek to Mount Sinai and their birth as G‑d’s chosen people.

In ancient times the Passover observance included the sacrifice of the paschal lamb, which was roasted and eaten at the Seder on the first night of the holiday.
This was the case until Temple in Jerusalem was destroyed in the 1st century.