Hi Ashley!
Fall is a wonderful time to visit Walt Disney World and Mickey's Not So Scary Halloween Party is by far my favorite special event! This year the party will begin on September 10th and run through November 1st, so that gives you plenty of dates to see the spooktacular decorations.
In my opinion, I would visit mid to late September for lower crowd levels. After Labor Day, crowd levels will slowly dwindle down which makes this a great time to visit. September also offers several money saving promotions such as
Disney's Free Dining and several Disney Resorts have substantial room discounts. September is during hurricane season so it is something to keep in mind, although I have visited several times in September and have never had a major issue with the weather as yet.
The first week of October is also another great time to visit as the crowd levels will still be low and Epcot's International Food & Wine festival will also be open. The crowd levels will slowly increase the rest of the month due to Fall breaks and Columbus Day vacations. Also be aware that the first weekend in October is the Tower of Terror Marathon weekend, which will draw runners. It will also cause Hollywood Studios to close earlier than normal on October 5th due to the 10K evening run.
When purchasing your tickets to Mickey's Not So Scary Halloween Party, I recommend to attend this party on a weekday as the weekends tend to be much busier. Tours such as Keys to the Kingdom, where you get a backstage look at the magic, are held throughout the year. You can make your reservations by calling (407) WDW-TOUR.
I hope you have a wonderful visit and please let me know if you have anymore questions.
Best Wishes!
Mandy