This is what has happened in past MVMCP and is happening now at MNSSHP.
Party guests are allowed to enter at 4 pm.
Day guests are gently herded to the exit at 6pm. No one without a wristband is able to get in line for any attraction after 6pm. Nor restaurants, shops, or counter service. Nada.
The official starting time for the party is 7pm. By then the day guests have left as eagle eyed CMs have been watching everybody for official party wristbands. They even check the bathrooms.
We have found (as we go often early in December) that on those short days people either avoid MK or leave at 2pm or sooner to hop to another park. Lines are generally shorter on party days as well. However the closer you get to December 22 (last party date) all bets are off.