Why do you think Twilight was named that? Do you think it is appropriate?

Well, your question got me curious, so I did a little research. The answer to your question was found on Stephenie Meyers website:

Twilight was not the easiest book to title. When I started sending out queries, I called it Forks for lack of a better idea. The first thing my agent advised me was that the title was going to have to change. We played around with a lot of different titles, and nothing seemed to convey the right feel. We brainstormed through emails for about a week. The word twilight was on a list of "words with atmosphere" that I sent her. Though these words were meant to be used in combination with something else, the word twilight stood out to both of us. We decided to try it out, and, after a little adjustment time, it started to work for both of us. It isn't absolutely perfect; to be honest, I don't think there is a perfect title for this book (or if there is, I've never heard it). When I look at the titles other countries have used (examples: Germany: Bis zum Morgengrauen which is "Until Dawn" or "Bite" if you add an "s" to make it "biss" (if you look at the German book cover on the Twilight International page); Finland: "Temptation;" France: "Fascination;" and Japan, which has split it into three separate books: "The Boy Whom I Love is a Vampire," "Blood Tastes Sadness," and "The Vampire Family in the Darkness."), it seems like I might be right about that. (Both New Moon and Eclipse were much easier to title, and the titles also fit better.)

I think Bella compares her life to celestial occurences, and her life is like twilight in the first book... I dunno.

It used to be called Forks, so I guess Twilight is kind of an improvement.

Kinda like the answer above...
I heard it was something to do with the stages of vampire-ness and the stages of darkness.

Or something...