Well, if you're looking for a large convention, there's Dragon*Con over labor day weekend. It's not so much a gaming convention though as a lot of science fiction/fantasy/fandom conventions rolled into one. There's RPGs happening, but they're dwarfed by the board gaming, CCGs, miniatures, and Twilight Imperium game (singular - one game of Twilight Imperium played from start to finish over five days) happening in the same area of the hotel. Plus the EFF, skeptics, YA, Wheel of Time, British media, American TV, costuming, etc. etc. etc. conventions happening in the other five hotels in the area. The dealers' room is enormous by the way. Two rooms technically. And by rooms I mean convention floors.
Dragon*Con is less a convention you do than a convention you plan which convention within a convention you're doing that year ahead of time. Which might be costume watching the whole con if that's your thing.