We live in one of those states that has criminalized attendance matters, and the number of permitted unexcused days is quite small (no more than 2 in a 60 day window), so we either tie things in with long weekends or times when school is out. On top of that, the job I took in 2014 makes it impossible for me to travel in Sept & October, the first half of April and the first half of June. My schedule was a bit more flexible before the kids and even before the kids were in school, plus I was an FL resident for a while, so I've experienced the parks at many different times.
Honestly, I know people always say the parks are so much more crowded these days, but we've been going since 1985 and I guess since we always went when school was out, I've always thought the parks were busy. Mid September and early January were the only times I ever thought the crowds were really light. It's funny, when you're used to going at crowded times, it's actually kind of creepy to see the parks dead. I know you get on more rides, but to me, some of the life and character of the parks is gone when there's a lack of patronage.
Ultimately, while I LOVE the holiday decorations and have been fortunate to go several times during the holidays...I like summer. I've lived up north and in the south and I've never understood the freak outs over the normal summer conditions. I've been in the parks when it's in the 30s and 40s and while some people laugh and say that's nothing...I'm sorry...the parks are just not fun to me when you're dealing with a wind tunnel on MSUSA. Summer just makes water rides and pool time better and treats like Dole Whips and premium bars are just that much more refreshing. And while it's not the case this summer, usually summer equates to longer hours. Room discounts are also pretty decent over the summer and while not dead, the crowds are never as bad as the crowd calendars say they will be.