Certain parts of May are a great time to go if you want warm weather (but not excrushiatingly hot) and lower crowds. The later you go in May the more the crowds will pick up as schools begin to get out for Summer. Try to avoid the last week of May and especially Memorial Day Weekend. On the front end of May, you might encounter Grad Nights at the Magic Kingdom during the first weekend of May, which just means the Magic Kingdom will close early on those nights. So, all this to say, probably the very best time in May, would be around the second week of May.
Good luck with your trip planning!
