April is nearly as hot as August and the crowds are *monster* during spring break.
It requires very special, careful planning to go during this time and you have to acknowlege that unless you are willing to wait hours for some rides, you may not get to ride many of them.
We were just there for valentine's, and were *shocked* at the crowd level. After we waited in line an hour for food at Flame Tree BBQ at AK, my husband was like, we need to go home, this sucks.
I agreed, it did suck, but the problem was that we should have looked for lunch at 11 am with no lines, not waited until 1 pm. We know better, we just didn't expect the crowds and hadn't created an itinerary to deal with them...
I say go in late July and do what we call The Vampire Trip: the parks are open late so you stay at the pool all day, take naps, and then only hang out at the parks in the early morning and then after the sun goes down. You never get sunburned, there are no crowds at 11 pm in the magic kingdom, and it's AWESOME. Our kids switched over to night time much quicker than we did-by the second night when MK was open until 3 am, we were chugging coffee like crazy but they were fine

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Also late July avoids the "holiday" crowds-all of europe and south america shows up at Disney's doorstep in August, and they can be tough to deal with at times because of different customs.