I just called and made my ADR for October 2nd which is one of the nights of the Halloween party. We got in at 5:45. I've read that you can get into the parks any time after 4 with your party tickets and they corral everyone into one area from park close to the start of the party. So while people are being corralled, we'll be having dinner and finishing up in time for the party to start.
Hope that helps!