If you ask a Jew today what the Passover commemorates, he would (in all likelihood) say that it commemorates the Exodus from Egypt. But is that what the Bible teaches? No, not at all. The scriptures define the meaning - because it was named for an event that God performed.
The day and the sacrifice are named after the event that God performed when He passed over the children of Israel. It does not memorialize, it does not commemorate, the going out of Egypt. It memorializes, it commemorates, God passing over (sparing) the Israelites.
The Passover is established to commemorate God's passing over. Nothing could be plainer - to anybody who is of a mind to believe God! We are dealing with two different events, two different days, and two different festivals - one on the fourteenth, and one on the fifteenth.
A different event altogether! They went out on the fifteenth. The Passover took place on the fourteenth. The Passover commemorates the passing over. The Feast of Unleavened Bread memorializes, and commemorates, the going out.