It's hot anytime the sun is out. But for a time when it starts dropping slightly in temp, around 6-7pm.
We are AP's so we normally hit the park in the morning, leave around noon or 1, then go back to another park around 6 or 7. We go pretty often, so we don't mind not staying in the parks all day.
But be prepared for the heat. The only time it's really not hot is when the sun is down..and even then it's still warm lol.
Like for tomorrow. At 1pm it's going to be 88 degrees (without humidity), it doesn't drop until 7pm when it gets down to a chilly 83 lol. By 10pm it's 76 and that's as cool as it'll get all night.