I wouldn't worry about it.
When the parks close at 6pm, that means crowds are light early in the morning.
Arrive at 08:30am, be first to the gate, take a lap of walk on rides, and you'll be fine.
At 4pm, go have some happy hour drinks at the bars in the parks before they close. Head over to a CityWalk restaurant to eat dinner, then go back to your hotel room to crash. Head down to the pool, and sit in the hot tub.
You'll miss whatever event is happening at the parks. You won't even know about it.
There are ways to avoid the crowds. Basically, you go to the parks early, then come back to the onsite hotel and have a great time.