Traditionally it has been held the Sunday night after Thanksgiving, so the night of the 30th. It also has been held the evening of the Friday before Thanksgiving, so the night of the 22nd. This has been the case for 3 years as I always plan my trip to capture the Party myself, so I checkout on the Monday after Thanksgiving.
But definitely keep an eye out for an official announcement. With Star Wars possibly opening up around that time of year I suspect all traditional time frames for these events will be out the window possibly.
This is definitely a good point. This is the first time I've ever seen one offered the Sunday before Thanksgiving. I know I would be buying if I was there.