When the parks reach capacity they close the gates, apparently there's a system if I remember correctly (from reading these boards) they close the parking first, then the entrance gates allowing only resort guests then complete closure ...i think that's how it works.
Although being a resort guest gives you the benefit of getting into a park...2 things to remember
1.) you'll get in A park, not necessarily the park of your choice
2.) if it's at capacity make sure you really really want to deal w/crowds/lines etc....
Once they close a park it can and usually does reopen throughout the day so get there early! By 11am the parks are packed.
Now with all that I went last yr Dec. 25-28th and.....
saw/did EVERYTHING except Test Track that I wanted to do!
We were at the parks at opening, break at 1pm back at AS Sports, then back to the parks til closing. Loads and Loads of people BUT I was celebrating Christmas w/the Mouse so nothing mattered!
HAVE A BLAST!