Official holiday celebration starts Nov 22. I've got pictures of the holiday decorations in Seussland from Nov 7 last year. I think they were still decorating the resort hotels, but the parks should be mostly done by that point!
Yes, most of the decorations in Islands are up by Halloween & most of the decorations in Studios are up about a week after Halloween. Just no holiday shows/meets that early.
Honestly they start HHN in August now - you would think they could start Grinchmas a bit earlier - I know that HHN is a $$$ ticket event but still.
I used to do Disney the week of Halloween and pretty much they were Halloween one day and Christmas the next - I used to plan my trip around that so I got both holidays in one trip.
With the Veterans day weekend and all they should be all in with Grinchmas at least the first weekend after Halloween or for veterans day.