We are going this year from the 23rd to Jan. 1st.
I went last year with just my DD-6. We arrived on Christmas Day and went to MK. We got to MK around 6:00pm and had dinner at Crystal Palace at 7:00pm. We had no issues getting in and we stayed until 3:00am.
We ate, walked around and enjoyed the decorations, watched the fireworks, and night time parade. Around 10:00ish it started to clear out. We were able to ride all three mountains, POTC, Buzz, Philharmagic, Small world, haunted mansion, plus a few others after 10:00 until 3:00am. The park was not at all busy from 1:00am-3:00am.
My DD is a night owl, we planned this, slept in that day and the next and we had a perfect unrushed day.