It's always a little bit of a gamble booking DC in Dec, Jan of Feb as I'm sure you know there is the chance of a cold snap hitting Fl. In general, if you get average temps, then you should be OK. Coming from the North East (as I think you are) I would book DC to be in the first day or so of the vacation, before your body has a chance to get used to the warmer weather. It should help to make the day feel a little warmer than a day of the same temperature that comes at the end of the vacation.