It’s not one set date, per se, as it takes time for everything to be in place. MK and DHS are usually first, due to the holiday party dates in early November (last year, the parties started on the 8th and 9th, respectively). Festival of the Holidays at Epcot doesn’t start until Thanksgiving weekend, so Epcot and AK are typically completed later than MK and DHS. Everything is usually finished by Thanksgiving week.
For MK, most of the decorations will go up the night of October 31 (into November 1). We always joke that Thanksgiving doesn't exist at Disney, they go straight from Halloween to Christmas. As PP noted, the rest will follow suit.