Usually anytime in May is a good bet, but the closer to the end of the month the more crowded it does become and also the warmer! I like the first 2 weeks the best, it is also a great time to experience the Flower and Garden Festival at EPCOT, the weekends for that tend to be a little busier with locals. May is a great time to visit the world, well actually anytime is! but the weather in May is usually perfect!
keiki,binky&mouse my 3 beautiful white kitties!
I'd pick one of the first two weeks. We got there May 21 and from that day until the 31st it just got busier (Memorial Day this year was 5/31). It was also very hot (high 90's).
We were there just this year from May9-15th and crowds were incredible!! Like Keddy said...most waits were less than 15 min. We walked onto many, many "big" rides. By Friday, the crowds did pick up though!! We LOVED going this week!