They went up a day or so ahead of the first
MNSSHP on our trip a couple years ago. We were at MK on 8/30...ran in to do a FP on 7DMR. Main street had decor up and a few shops. We attended the first MNSSHP that season on 9/1 and there were a few more things up but I believe they were for the party. Such as fog and lighting etc. The decorations were not over the top but for sure made it feel special (even Christmas has been cut way back IMHO.)That said, it was our first time seeing WDW with fall/Halloween decorations and I love it. Halloween is my favorite holiday though, lol.