We went on 10/25 and it was very hot! I wore a cape I had sewn for my random fairy godmother costume, but a t-shirt and shorts. DH wore a t-shirt that said "costume," and DS wore regular clothing. Remember that you may be climbing in and out of rides. When we received our tickets by mail, the letter included some advice about nothing that drags on the ground.
I is a great event. We will be there again with DS, DD(playing a little hooky from college) and DH. No costume themes yet.