This feast remembers the last plague in Egypt when the angel of death "passed over" the children of Israel who applied the blood of the lamb to their doors. The Israelites took a bundle of hyssop and dipped it into the blood in the basin at the threshold. When John the Baptist said, "Look! The Lamb of God who takes away the sin of the world!" he understood the Old Testament reference.

In the New Testament, we see that Jesus - born in a stable, visited by shepherds, and led to the slaughter - is that lamb sent for us. His death allows the judgement we deserve to pass over us.