If I had better options, I would NOT choose to go in the summer.
But our last trip was in mid June and not only did we survive, but we also had a great vacation.
Now that we have kids in high school, the only choices we really have are:
1. Easter week
Weather is perfect
Crowds are crazy
We only get 7 days there.
2. Christmas week
No heat, but could be chilly, so pools and water parks might be out
Crowds are even crazier
3. Summer
Brutally hot
Crowded(depending on which month), but not nearly as crowded as first two options
We can do a longer trip
We chose summer for our last trip for a few reasons:
We don't like long lines and huge crowds.
It allowed us to spend two weeks there.
My top summer survival tip is to stay out of the parks between 1pm and 5pm. Even though it's hot 24 hours a day, the mornings and evenings are more tolerable.
On most days, we did a morning at the park, then went back to our rental house and relaxed and let the kids swim. Some days we went back to the parks for the evening. With longer summer hours, this is easier to do. Then on some days, we slept in and didn't even go to a park until evening. Being able to go for 2 weeks made it easier to do some half days without missing anything.