As Safetymom stated as a resort guest you are guaranteed entrance to "A" park. It may not be the park of your choice. Here are the stages of closing just for reference... you will see that resort guests are the last ones that are turned away but yes it can happen and usually does the week of Christmas and New Years (as well as Spring Break/ Easter).
Phase One:
Guests meeting these criteria are told the Park has reached capacity.
Day Guests with MYW Base tickets or one-day one-park tickets, Guests without Theme Park admission and Cast Members will be turned around at the Auto Plaza.
Phase Two:
Only guests meeting the following criteria are admitted: 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.
Phase Three:
Only guests meeting the following criteria are admitted: 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.
Phase Four:
Closed to all Guests, the Park has reached capacity.