We did MK EMH for the first time this past February. We arrived at around 7:00 pm and it was insanely crowded. However, after the fireworks ended, and people realized they couldn't get on the rides without a wristband, the park cleared out pretty quickly. It probably won't be that bad in September for you. We found that Adventureland was never crowded (not too much open there), so it was a nice respite while waiting for the crowds to die down (around 10-ish). By 11:00 pm or so, there were no lines at all and we were just getting off/getting right on again for rides. It was totally cool riding Dumbo at midnight!!