Speaking as a guy who's favorite holiday is Halloween, my traditional time is Oct. 1-ish. I guess if it's general autumn decorations then maybe mid-to-late September is fine.
Marginally related, I once remember seeing on this TV show, some guy who celebrates Christmas everyday; eats like it's Christmas, opens a present, and I presume keep the decorations up year round. Whenever that guy comes to my mind I try and imagine what the Halloween version of him would be like.