According to Exodus 12, each household selected a lamb before Passover.
The Passover instructions required each Israelite household to select a male lamb without blemish, one year old, on the tenth day of the month. It was kept until the fourteenth day, when it was slaughtered at twilight. Its blood was placed on the doorposts and lintel, and the meat was eaten with unleavened bread and bitter herbs.
Exodus allows a household that was too small for a whole animal to share with a neighboring household. The animal could be taken from either the sheep or the goats, although the familiar short answer is lamb.
This ritual meal is linked to the final plague and the Israelites’ departure from Egypt. It is distinct from the later Festival of Unleavened Bread, though the two observances became closely associated in Jewish tradition. The lamb’s blood marked the Israelite houses for protection in the narrative.