Kids go back to school August 20th. If you come before this it's going to be packed. If you come after this, the place is a ghost town, but closes early at 8pm instead of 10pm.
Here's the secret if you are coming before August 20th. The parks will open early, at 08:30am, yet they don't tell anyone. Therefore, you want to arrive the parking garage at 07:45am.
Some like to sleep in on vacation and arrive the park at 11am. Don't do this, that's what 90% of the people do who visit. There is a herd of people who show up at 11am all wanting to do the same rides. Do the early thing and you'll have a great visit.