The summer is pretty crowded no matter what, but late May before schools are out is good. But then that's really not technically summer...never July if crowds bother you because that's when the tour groups go! The summer is crowded but if you plan well then it's not a big deal. Make dining reservations in advance, do online check in, make sure you use FastPass, avoid parks with EMH or events like Star Wars Weekends, and you'll be fine!