As Michael said, 4pm, but, there will be no FP+ available after 7pm and if you only have a ticket for the party then you won't be able to book them at all, but, I would do my ADR between 4pm and 6pm anyway as I wouldn't want to waste a moment of the party sitting inside eating, I'm not even going to be doing too many rides as there is so much going on with the Trick or Treating and lots of photo ops with characters who only come out on special occasions like parties.