As a general rule I usually pick out a decor I like, and I leave it alone. I try not to swap it around for holidays since I put so much effort into getting the layout just right that I don't want to have to get it all back together once the holiday is over. Then again, I usually have super dense layouts, so someone that sparsely decorates would probably be more willing.
That said, certain holiday decorations might be used year round. I heard about someone who set up their tank looking like a desert and put glass catfish in it so the tank looked like it had skeletons flying around the desert... I could see Halloween decor being particularly popular for year round decorations on a similar design. Monster fish keepers may get a kick out of skeleton, bones, ghosts, alien and monster decor for their monster/oddball fish to hang out with. I don't see Easter, Valentine, or Christmas decor being quite as popular...