1 month may not have mattered too much a few months ago, but many schools are now at the point where they have probably interviewed at least half of the total amount of people they plan on interviewing, if not more. So at this point, one month may mean the difference between being interviewed for a position that is still available or a position on a waitlist as schools fill their inital class size.
I would definitely advise you to go in December unless it would be totally impossible for you to do so, even if it would cost you some extra money, especially if its at a school you would like to attend very much.