This is going to be one of those ones that make me smack myself because I forgot, I just know it.
Season 4-ish; the boys are roommates, but Xander is crushing on Spike. Spike goes to LA to a club where he runs into Angel, and ends up crashing at the hotel. Eventually it is Xander's birthday (I remember there is a birthday party in the alley behind the magic shop), and Spike takes him back to that same club, where he finds out about Xander's crush. Xander runs away to Angel's hotel, where Spike finds him.
I know it's not much to go on, but does this sound familiar? I cannot for the life of me remember the title or the author.