I can't speak for all the churches, but in the catholic church it's the day traditionally set aside to honor three different saints, all named Valentine. One of which was supposedly martyred on the 13th. Though actually on the modern church calender it's no longer celebrated, unless you're very traditionalist. Or it could be just some day to give flowers and candy to your lover. Of course if you need a holiday to remind yourself to do that, I think you have a lot of problems.
Toxic, happy birthday,
I've gotta say, I like your way.
When the topic is stupid, you act intelligent,
But when it's smart you act without sentiment,
That is why, you're one of the funniest people here,
So here's a toast, to another year!