December weather in Florida is just as unpredictable as what you see in Illinois. How was this December compared to other Decembers?
Like Diane, we've done numerous December/January trips and we've seen times when it was below freezing at night, and barely into the 40's during the day, and other times when it might be 60 at night and 80 during the day.
About a week before your trip, the long range forecasts will probably give you a fairly decent idea of what you can expect.