Holy Thursday is sometimes called Maundy Thursday, from the word mandatum, "mandate", referencing the order Jesus gave His disciples after washing their feet.
At my parish, the feet of 12 men are washed, and it's usually the Dominican community (we currently have four priests and one cooperator brother in residence, but we usually get one or two extra with us for the triduum), the altar boys (we only have altar boys), and, if that's not enough, the lector or another man from the congregation. The pastor then washes one foot of each man, while the choir sings. It's done reverently and relatively quickly (meaning it's not a slog to go through--everyone's got this down to a science, by now).
Of course, this Mass also celebrates the Institution of the Eucharist, which is the focus of Mass (which is the "source and summit" of our Catholic lives). Our Eucharistic beliefs are really one of the richest parts of Catholicism for me.
My freshman year in college, I was a member of the debate team. And my debate partner (there were two people on a "team", so our team had a few different teams) and I would debate a lot more than just our assigned debate topics, like famine in the Horn of Africa. He wasn't Catholic, and he had questions about Catholicism.
One night after practice, we went to the campus library and headed to the second floor, where the Bibles were kept. We laid them out on one of the tables and went at it for a few hours, until the library closed. (This is the sort of thing I like doing, by the way.)
Proofs for Transubstantiation aren't hard to find; John 6 immediately springs to mind. I brought that up with my partner. "It's just a metaphor!"
"Jesus knew when to use metaphor and simile. He does it all the time. The Kingdom of Heaven is like a mustard seed. Or a fine pearl. But he doesn't do that here. He's pretty explicit. And wouldn't He have had to clarify his remarks, since, to the Jews he was talking to, He's suggesting something crazy radical? Jews don't have anything to do with flesh and blood together. They're freaking out here. But Jesus doesn't say, 'wait, you guys, you're wrong! It's a metaphor!'"
"But that's what it is. It's just bread and wine!"
If it's just a symbol, than to hell with it (as Flannery O'Connor said). To Catholics, the entire Mass is built around the Eucharistic sacrifice--the moment of transubstantiation. (This means, by the way, that when the bread and wine are consecrated ["This is my Body", "This is my Blood"] they become the Body, Blood, Soul, and Divinity of Christ, even though they look like plain old wafers and wine.
I love the Eucharist, so I generally love Holy Thursday Mass, because we're celebrating the institution of that Sacrament. Not only is it theologically rich, but we also get to sing one of my Favorite Catholic Songs, the Pange Lingua. (Written by a Dominican, incidentally--Go Thomas Aquinas!)
In the triduum, Mass doesn't "end" the way it normally does--the Triduum Masses/services are all one big liturgy. So after the Prayer After Communion, the Sacrament is taken, in procession, to an altar of repose, usually decorated to resemble a garden (like Gethsemane). The Eucharist isn't reserved in the Tabernacle--the Tabernacle is empty, and the sanctuary lamp (the red candle) that is usually lit, indicating the presence of Christ in the Tabernacle, is extinguished.
At the Altar of Repose, you can pray in silence until midnight, when no more solemn adoration is allowed, until after Easter. I usually stay for about a half hour, reading the Bible and a few other things. This year I read John 14-17, the Great and Final Discourse of Jesus at the Last Supper, and I was overwhelmed with the way certain things spoke to me; so much so that I decided it would be my lectio for the coming days, and it has been. It's so theologically rich. I'll be sharing those notes with you later.
(The Adoration is us staying awake with Christ in His agony--doing with the disciples couldn't do, that first Holy Thursday night.)
The Gift of the Eucharist is one of the supreme gifts of Catholic life. I love this Mass that celebrates it, and kicks off the triduum.
The Eucharist, as Christ's saving presence in the community of the faithful and its spiritual food, is the most precious possession which the Church can have in her journey through history.
--St. Pope John Paul II, Ecclesia de Eucharistia, April 17, 2003