There are actually Four stages to a park closing:
Phase One:
Guests meeting these criteria are told the Park has reached capacity:
Day Guests with MYWBase tickets or one-day one-park tickets and Guests without theme park admission will be turned around at the Auto Plaza.
Phase Two:
At this time, admission is limited to Resort Guests, Annual and Premium Passholders, Guests with Park Hopper tickets, and Guests re-entering this Park today. Guests using WDW Resort Transportation and Guests in taxis will be admitted. Guests not meeting the above criteria are told the Park has reached capacity.
Phase Three:
The theme park is only open to Resort Guests and Annual and Premium Passholders. Shades of Green and Downtown Disney Resort Area Guests using their respective transportation will also be admitted. Guests not meeting the above criteria are told the Park has reached capacity.
Phase Four:
Closed to all Guests and Guests are told the Park has reached capacity
In the past 3-4 years, they have been allowing WDW resort guests to leave and come back to the same park. Say if you are at MK from opening until around 3-4pm and decided to go to the hotel and take a nap, you would be able to return to MK later in the night because that is the park that you initally used your ticket at for the day. If you were to leave Epcot, take a nap and try to go to MK they would not allow you in (if MK is at capacity) because you had initally used your ticket at Epcot. Again, there is no Guarantee that they will let you back in once capacity has been reached (which on Christmas Day will be VERY early in the morning) but this has been the practice that they have followed for the past few years.