My family goes Christmas week - December 24th through January 1st. This past year (2013/2014) the weather was perfect. It rained one day - but that's to be expected.
I can tell you this - in Florida it's relatively dry during the month of December. On average, it rains 5 days. We had one day where it was cloudy and on-and-off small showers and one day where it POURED.
As for the weather - we arrived in 2009 (I believe) for our first trip. I remember my grandma saying how cold it was (she lives in Tampa.) We laughed - this was coming from a woman who complains that it's "too cold" when the air conditioner is at 70 degrees in the summer! Well, the joke was on us. We arrived to 70-degree temperatures and sunny skies. By the time we checked in and settled in our room, the sun was gone and the temperature dropped into the 50s. It would not reach the 60s the rest of our trip. In fact, the temperature would sit in the low 50s during the day and dip into the high 30s, mid 40s at night. It was FREEZING.
The next year we were more prepared, but we were met with similar temperatures. I remember on NYE Day it was around 65. That was considered a "hot" gorgeous day to sit outside by the pool.
We skipped 2011/2012 and went again in 2012/2013. The weather was much better in 2012/2013, with only two or three days dropping into the 50s and those two nights being high 40s.
This past year the weather was perfect. The temperatures hung around the 60s. We had two or three days when it was in the 70s. For us, we were comfortable in jeans and a t-shirt when it was "hot."
Like I said, we had two days where it rained. On those days it was noticeably chillier out. If it's a cloudy day, it's going to be cold. Florida is a damp, humid place. In the summer, that means hot and sticky weather. In the winter, that means that when it is not sunny, there is going to be a damp chill in the air.
Aside from the weather varying, so does one's point of view. In 2010 we were freezing. I remember being bundled up and meeting a couple on the bus that were wearing shorts. We got to talking with them and they were from Alaska and enjoying a "tropical Christmas."
Let's HOPE that 2014/2015 resembles the weather from this past year and not from 2009 or 2010. You basically need to pack for multiple seasons and hope for the best. It's better to overpack and not use clothes than underpack and wish you had more clothes!