Ive been 3 time at the end of December. 2 of the times it was 80-85 degrees during the days everyday, with morning temps about 65-70. Absolutely beautiful!!! We go tin the pool to relax, took naps by the pool, and wore summer clothes during the day, the Girls wore Fall clothes at night once the sun went down (they were all cold natured). But being from Penn, I don't think the nights will be a problem for you at all.
But that was 2 of the 3 weeks. The other week they had record cold weather. Highs around 40 and lows in the upper 20's and lower 30's. VERY cold for WDW.
But that was record cold weather. In fact 3 of the 5 days we were there had record lows. More then likely though the weather will be GREAT. Highs 75-85 and lows 60-70. In my opinion it is the best time of the year to go weather wise.