I will pile on with June. We've done both several times, and as mentioned several times, June is not bad if you are prepared for the heat. In the last couple of years, in fact, early Summer has been less crowded than in the past.
Have a good plan. Rope drop with midday break, and come back to close down the park. You will be maximizing your time during the least crowded periods, and also avoiding the hottest part of the day.