The kids are out of school.
The most busy time of the year is July 4th - August 20th.
During this time of the year, the parks will open at 08:30am but they don't officially say anything about it, just happens.
Therefore, I tell everyone to arrive the parking garage at 07:45am, and there will be a short line of cars doing the same thing. Park, go straight to the gate and wait in a line. If the park open at 08:30am, you'll can go on the most popular attractions with no wait. 11am is when the "heard" shows up, the parks become packed, and it gets burning hot with the sun overhead. Do a couple indoor show attractions if the line isn't that bad, then leave, go back to your hotel, rest, and visit another park at night.
This plan can be done any day of the year. The key is to be at the gate early.
If you sleep in late and arrive at 11am, forget it. That's what the other 99% of the people who visit do, they all show up at the same time. They all sleep in because they're on vacation, take their time strolling out of bed, go to breakfast, run late, and it's 11am. Nope, you have to wake up at 6am like it's a work day, eat doughnuts in the room, and get going.