There will definitely be phased closures.
Last year, Magic Kingdom saw a Phase 3 closure on Christmas Day, December 26th, December 30th and 31st.
If the park closes at 11am, it's very possible that it will re-open by the afternoon.
If you are staying onsite, you should not have a problem getting into the parks. My family has entered parks as late as 10pm on NYE and never had a problem. That's not to say that this year won't be different - I may be eating crow this year. But during all of our trips - no park has reached a Phase 4.
For more info on Christmas crowds and how to plan your days, check out my free guide at suvivedisxmas.com