The crowds start dying down at the end of August (except for a predictable resurgence on Labor Day weekend, being the traditional last blast of summer), so you'll have a good day no matter what day you pick, even with the short hours. We're actually planning to send our whole company up about that time, assuming this month goes well for us. The timing of the trip is very deliberate; schools are back in and the day crowds become much lighter, so we have a good chance to hit the entire park. Like Gemini said, just pick the day that isn't raining :)