In the Christian tradition, Jesus is referred to as the "Paschal Lamb" or "Lamb of God" because his crucifixion echoes the ritual sacrifice of the Passover lamb. Many aspects of Christian symbolism have roots in the Jewish faith. In the Jewish tradition, "Paschal Lamb" refers to a lamb sacrificed and eaten as part of the rite of Passover.
Passover commemorates the deliverance of the ancient Hebrew slaves from a plague wherein the firstborn of every family would die. God instructed the Hebrews to slaughter a lamb and paint their doorway with its blood. This lamb's blood saved the family from the death of their firstborn. The killing and eating of "a lamb without blemish" became a part of the Passover ceremony in memory of this story.
Many aspect of this Passover story appear in Christian symbology around Jesus, who is described as "shedding His blood to save the world," a "Lamb of God" or "sacrificial lamb," and "without blemish."
Passover takes place at roughly the same time as Christianity's Easter, which celebrates Jesus' resurrection after his sacrifice.
In the Catholic tradition, the practice of Eucharist, the ritual consumption of bread and wine to symbolize the body and blood of Christ, also echoes the Passover ritual of eating the sacrificial lamb.