"You had to eat the lamb"

What, exactly, does Holy Thursday commemorate and celebrate?

The Last Supper, yes.

The washing of the feet, certainly.

The Institution of the Eucharist, of course.

But for deeper insights into the Eucharist, we go to Scott Hahn:

Now you can't eat a lamb cookie in Egypt. If you didn't like lamb, you couldn't have your wife make lamb bread, little biscuits in the shape of a lamb and say, "God, you understand, we just can't stand the stuff." No, you do that, your firstborn would die. You had to eat the lamb. Jesus Christ has said to us, "My flesh is food indeed and my blood is drink indeed. He who eats my flesh and drinks my blood has everlasting life."

Read more at "The Institution of the Eucharist in Scripture", from a talk by Scott Hahn.