What is the Passover? Right from the start, God knew that young people would ask this very same question: "And it shall be, when your children say to you, 'What do you mean by this service?'" (Exodus 12:26). So He prepared an answer for them: "It is the Passover sacrifice of the LORD, who passed over the houses of the children of Israel in Egypt when He struck the Egyptians and delivered our households" (verse 27).
Passover is a memorial day—a very important anniversary day. However, it commemorates three events, not just one. As God said, it commemorates the tenth and last plague upon ancient Egypt in which, after giving them ample warning, God passed over the nation of Egypt and killed all the firstborn in the land. Through this decimating plague, God freed the children of Israel from their captivity and servitude in Egypt.
Secondly, and most importantly, it commemorates the death of Jesus Christ, who was and is the firstborn Son of God the Father. Through Jesus' awful death—which, by God's design, took place on Passover day in AD 31—God freed us, regenerated Christians, from our captivity and slavery to the world, to Satan, and to sin.
Finally, it commemorates the baptism of each Christian, when we formally accepted the death of Jesus Christ, when we asked Him to apply His priceless sacrifice to our sins, when we asked that He would cover and blot out our sins with His blood (Psalm 41:1, 9; Acts 3:19; Romans 4:7).