I don't think it's been announced when halloween time officially starts this year. If I remember right the start date is usually somewhere around the 10th, but varies year to year. So you will probably see some decorations, but not necessarily everything if the season hasn't officially started yet. The rides with overlays likely won't be open before the official start date. With Halloween being your primary reason for going, I would pick a later date to be safe. If you want to catch a Halloween party, those start later in the month.