We were in WDW from the 16th until the 29th December last year and we quite a mixture of weathers.
A few rainy days
A few cloudy days
A few days of sunshine.
and some quite windy days too.
Temperature wise, mornings were generally chilly, low 50s to mid 60s.
During the day temperatures ranged from mid 50s to high 70s. On the warmer days it was very nice in the sun.
Late afternoon temperatures tended to fall most days and once the sun had gone down it got quite chilly. There was the odd night were it did stay mild, but not really warm enough for shorts and t-shirts. There were also a few nights that were very cold, especially when it was windy as well - on these nights it was sweatshirt and coat weather. If you are feel the cold then even a hat and gloves wouldn't be over dressing.
During the day you need to wear layers that are easy to remove/put on. You can always dump excess clothing in the park lockers.
Evenings you will need at least a sweatshirt and possibly a coat or jacket too.