Hello Rebecca,
September is a great time of year to go to Disney World. The weather's fantastic. I have found crowd levels to be really manageable. And on many nights, there's
Mickey's Not So Scary Halloween Party (MNSSHP)! It's my favorite time of year at the Parks!
Even though the party does not officially start until 7 p.m., in the past, Cast Members have permitted Guests with tickets for MNSSHP to enter as early as 4 p.m. And yes! Costumes are allowed and absolutely encouraged.
As you can guess, costumes aren't just for the little ones. I always go dressed up. But you have to remember, Disney is a family-friendly place. So costumes should not be offensive in nature. With that in mind, there aren't many other rules. Think of what you would normally do for Trick-or-Treating in your neighborhood: wear a costume you can easily walk in, make sure you can see through your mask, and hold on to and keep track of your Lightsaber.
The only other rule for the night is to have fun! Enjoy!