September always has some of the lowest crowds all year in Disney World since kids have just gone back to school and parents are reluctant to take them out at the start of the school year.
The first half of November is extremely light for crowds, especially during the week. This means low wait times and room to breathe in the parks.
December is certainly one of the most popular times to visit Disney World, with holiday decorations up and plenty of holiday-themed events happening.
Crowds disappear after New Year’s week which means excellent deals on vacation packages, hotels and park tickets.