From First Corinthians by George T. Montague, SM, commenting on 5:6b-8:
When Paul calls Christ the paschal lamb, or Passover lamb, he is evoking the entire conceptual and emotional world connected with the Jewish feast of Passover (pasch; Hebrew pesakh), of which Christ is the fulfillment. Even today Passover is the most important Jewish feast of the year. It evokes the entire story of Israel’s enslavement in Egypt, the plagues, the night of liberation when the Israelites slaughtered the Passover lamb, sprinkling its blood on the doorposts to save their firstborn from being slain, the meal they ate in haste with unleavened bread, the escape through the sea to freedom. The New Testament applies these images to various aspects of what it calls the paschal mystery: the Passover fulfilled in Christ’s death and resurrection.