Well, most of the sites start decorating pretty much from the beginning of October on. Although, you'll find most of the sites decorated during the couple weeks leading up to Halloween.
As for the Fort itself, they don't do much decorating, not like the Christmas decorations or anything (which around the Fort are pretty sparse anyways). Since so many sites decorate themselves, I guess they figure they don't have to put much up or something.
So yea, if you go at the beginning of October, you should already be seeing some sites with the decorations up. You defintely won't see any decorations done by the Fort itself yet at that point (like I said, they really don't do much for Halloween anyways).