Gb,
Memorial Day weekend (May 25-27) is usually pretty lively. There will be a poker tourney and a get-together of some kind I'm sure. There's a Roll Call thread up already.
Other than that, anytime is the best time to go. The "rainy" season is not bad at all. It usually rains a couple of hours in the afternoon. The rains are usually pretty light, but it can come down hard too. On one of my last rainy season trips, it only rained 2 out of the 6 days.
But think about it, most "activities" that you will partake in are done indoors (in your room or mp), unless you're into the outdoor thing

, so why would you even care if it's raining. The last thing you'll be thinking about over there will be the rain.