The tenth plague sent to Egypt was the sudden death of every first-born male, both human and animal.  This caused Pharaoh to release the Hebrews to worship in the wilderness, taking their families and animals.
God gave specific instructions as to how the Hebrews were to avoid the death of their first-born males. On the 10 th day of the month of Nisan they were to take a lamb without blemish. It was kept (prepared, sanctified, cleansed) through the 14 th day. As the evening of the 14 th day approached, they were to kill it, catch the blood, and roast the lamb over a fire. When the 14th of Nisan began at sundown, they were to brush the lamb’s blood on the door posts 

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