Had a very interesting conversation with a cast member today.
After speaking with a couple guest service managers and some others "in the know", Disney World is trying something new this year with the closing hours on party nights.
The party will still run from 7pm-12am, and the regular park hours will still be open to 7pm. However, on the computer system the park hours will stay open to only 6pm.
The change this year to make sure no dining reservations or fastpasses are scheduled after 6pm. This gives those that have dining and fastpasses the time to eat and ride before the party begins. The person did say the rides will still be open till 7pm, of course. The person said this was changed to help with the amount of people with dining reservations all the way up to 7pm, and then still being in the park after the party started.
Well, this would finally explain the posted MK park hours of 6 PM. A few comments relating to subsequent posts regarding FP's.
In prior years, the FP+ system would cut-off at 6:30 PM, 30 minutes prior to official park closing, but not until after folks had made FP+'s with a window that extended past 6:30 PM. This lead to FP+'s being cancelled/revised. Those of us in the know knew to make FP+'s that did not extend past 6:30 PM.
If the official park closing is kept at 6:00 PM, then the FP+ cutoff will likely not only be set at 6:00 PM, but (and this is the key) will also remain set for 6:00 PM. This means that WDW will avoid having to cancel/change FP+'s, as the correct cut-off was always in the system, based on the park hours.
However, if the cut-off time is 6 PM, I believe it will still be possible for party goers to get in 3 FP+ reservations. Last year, we had FP+'s windows of less than 1 hour at the end of the night. For example, 9:15 Pm - 10 PM for Monsters Inc. Laugh Floor, when the attraction was closing at 10 PM, and there were other I cannot recall. Assuming the FP+ system will offer 5:30 PM - 6 PM or 5:20 PM - 6 PM FP+ windows on party nights, then party goers would be still be able to have 3 FP+'s they can use, but the third window would be shorter (ending at 6 PM) than in the past.
Regarding the 6 - 7 PM hour in MK, I would expect that shortening the FP+ window by 30 minutes might increase standby times, while day guests leaving around 6 PM based on published park hours would reduce standby times. No way to guess in advance how these factors will balance out.