My guess would be early June and late August will have slightly smaller crowds. I've been there in mid-August, late-June, and early-July, and based on my experiences both the crowds and the weather were best in August. By weather, I mean, it rained the least, but it was definitely hottest (this was in'95). Wait times for attractions seemed shorter at that time, as well (and that was before Fastpass). Each of the last 2 years I was there in June and July and out of the 14 total days those 2 years, I'd say it rained about 8 of them the entire day. In my opinion, the crowds and the heat won't put a damper on a good time, but pouring rain can defintely take some of the fun out of things.