Help, I'm fashion-impaired

When is the usual time to shop for prom dresses? I've never actually been to a prom/formal/semi-formal, just weddings, so I'm looking forward to the night out in May. (By that point, those in my program will have really deserved it.) Hubby has said he will come along; not sure how much he will enjoy it but he's my man and the one I want to dance with :). The fact that there is a chocolate theme to the prom doesn't hurt things any (y'all know my sweet tooth :)). I'm looking forward to having a little bit of fun with my fellow MSTers...and wishing it weren't so far away (especially as that means I still have several classes to go between now and then).