Costumes are permitted during the Mickey’s Not So Scary Halloween Party. Costumes on children are allowed every day. At other times, for adults, you must not where anything that is SO close to a Disney Character costume that you may be mistaken for a Disney cast member. They also strongly dislike it if you allow pictures and start signing autographs. If the latter happens, you will probably be asked to leave the park. I know some people have been turned back at the gate whose costumes might be confusing to other guests as to whether or not they were Disney authorized cast members. They were asked very nicely to go change and they did. The costume must also not impede your own movements, so that you can move freely through the park without the aid of another person, for instance, you can’t wear something that covers your face so much that you cannot see to move on your own.