This is one of my favorite holidays - it gives me an excuse to eat chocolate (okay,I don't really need an excuse, but when one comes along I won't reject it).
I'd heard something about Valentine being a saint, and I'm not inclined to celebrate saints, so I went looking for the origins of Valentine's Day. Barbara Walker says in her book The Woman's Encyclopedia of Myths and Secrets that the original Valentine's Day was Rome's Lupercalia, a time when young men chose partners for erotic games by drawing small papers with women's names on them. Christians considered these valentines as "heathens' lewd customs." Churchmen tried to get people to put names of the saints or sermons on the papers but it didn't catch on.
February was sacred to Juno Februata, Goddess of the "fever" (febris) of love so the church replaced her with a mythical maryr, St. Valentine. They had trouble getting the story straight on him though, some said he was a handsome Roman who had been executed just as his sweetheart received her valentine.
During the Middle Ages, St. Valentine was much invoked in love charms and potions, since he was a Christianized (and therefore acceptable) version of such love-gods as Eros, Cupid, Kama, Priapus or Pan.