You may remember that the children of Jacob had gone from Canaan into Egypt to buy food in a time of famine.
There they found their brother Joseph, whom they had sold as a slave, who now was the greatest lord in Pharaoh's kingdom.
Joseph was very kind to his brothers, and Pharaoh welcomed them and their father Jacob to stay in the land, and be nourished there.
But by and by there arose a king who did not remember Joseph, and he began to put burdens on these Children of Israel the Pharaohs, one after another, making them into slaves, till at last their bondage was so cruel that they turned to the Lord their God, and cried to Him to deliver them.
He raised up Moses; and at length there came a night—that wonderful night—when the whole multitude of the Children of Israel were delivered right out of Egypt!
God told Moses that on this night He was going to send a destroying angel through the land of Egypt, and that the firstborn of all the Egyptians and of every living thing was to be destroyed, as a great judgment.
But God had provided "a way of escape" for the Children of Israel.
He told them to take a lamb for each of their households, and to kill it and roast it and eat it that night in all their houses.
God told them that when they killed that lamb they were to save its blood in a basin, and take a bunch of hyssop and sprinkle that blood upon the lintel and on the side-posts of their doors; and He promised that if they did this, He would not allow the destroying angel to come near that house, and they would be perfectly safe. God said, "When I see the blood I will pass over you."
And this was why it was called "The Pass-over."