When shop for prom dress?

It's probably not too early to start looking. This time of year they sometimes have good sales when you least expect them. Of course, buying a dress this early would depend totally on whether your weight stays fairly constant most of the time and doesn't fluctuate much. If you got one now, perhaps it'd be a style that would allow for minor alteration if it was needed, which could be done a month before the actual prom. If you do get it early, and you get it at a dress shop, I would ask if there is room for alteration if necessary. I never got to go to a prom when I was in school, but I have altered many dresses, and made prom dresses for my friends daughters.