During the week after Christmas up until New Year's Day, Magic Kingdom, Epcot, Hollywood Studios, and Animal Kingdom are subject to reaching capacity. Once they reach capacity, they will use phased closing to let people know that are arriving at the Auto Plaza that the park is now closed due to capacity. Walt Disney World uses 4 phases.
Phase 1 - When a park has reached Phase 1 closing, Guests with Magic Your Way Base Tickets or 1-Day 1-Park Tickets, Guests without Theme Park admission, or Cast Members using Main Entrance Passes or Silver Passes to get their families into the parks will be turned around at the Auto Plaza for the park they are trying to visit. Guests that meet the above criteria will be told the park has reached capacity.
Phase 2 - When a park has reached Phase 2 closing, Theme Park admission will be limited to Disney Resort Guests*, Annual and Premium Annual Passholders, Guests that are re-entering the park in question today, and "Free Admission on Your Birthday" ticket holders and their guests. All guests not meeting the above criteria will be told the park has reached capacity and will be turned away at the Auto Plaza.
Phase 3 - When a park has reached Phase 3 closing, Theme Park admission will be limited to Disney Resort Guests*, Premium and Annual Passholders, and "Free Admission on Your Birthday" ticket holders and their Guests. Guests not meeting the above criteria will be told the park has reached capacity and will be turned away at the Auto Plaza.
NOTE: As of 10:00 AM on December 31, Magic Kingdom has reached a Phase 3 closing. The criteria listed above are allowed entry to the park, as well as Guests that have ADR's at various CSR's around the park, as well as Guests with appointments at Bibbidi Bobbidi Boutique and/or The Pirates League.
Phase 4 - When a park has reached Phase 4 closing, the parks are completely closed to all guests and guests traveling to Auto Plazas will be turned away as well as told the parks have reached capacity.
I will be updating the thread today when and if any Phases have been announced for the other parks.