We have been there the first week in May and it wasn't very crowded. I would say to go the first or second week. After that time, a lot of schools get out and the crowds will increase.
We went the first week of May this year and it was not too crowded during the day, but the afternoons/evenings were really crowded. The wait times at GRR and Soarin were 60-90 mins and the FastPass line looked like it was just as long.
Thank goodness we had done all the rides we wanted in the a.m. when it was not crowded and we could just relax in the evenings
I would think lots of refurb goes on in may is that true? I went in May once but I can't remember what was closed.. I know matterhorn was.. and maybe splash?
I was there the week before Memorial Day and the only ride closed was It's A Small World. Everything else was open.
Saturday was a zoo, which I expected it to be, but it was so hot and crowded that it wasn't much fun. Sunday wasn't as bad, but we got there later in the day. Monday and Tuesday it wasnt crowded at all and it was perfect.
We went the 10th-14th (Sat-Wed) this May, and it was perfect... busier on the weekends (expected it to be) but really un-crowded, we rode everything so many times without a wait it was amazing... if the line said 10 minutes or more we skipped it and came back when it was walk-on...
I'd never imagined how many rides we were able to get on... on Sunday morning (we slept in and didn't do MM) we arrived 10 minutes before the regular park opening, and used the MM entrance and headed straight for Pirates... rode: Pirates, Jungle Cruise, Splash, Pooh 2x, HM, Tiki Room, dm went on IJ while dd and I did the treehouse... and then we looked at each other shocked it was only 9:45! In an hour and 45 minutes we had finished what we expected to be doing until lunch... so we headed for DCA and did a ton of stuff including Soarin and Monster's Inc over there! All before lunch...
We had a Grad night during our stay, but it didn't affect anything because it started after the parks regular closing time. However, don't know if that's always the case.