While just as sunny and equally palm treed as the rest of the state, Northern Florida has seasons. (A local joke maintains that these include 49 weeks of summer). Using Jacksonville's averages as a reference point - a January day might be 68°F, whereas you'll look for the ice cream van in July as it nears 90°F
While winter doesn't mean a white Christmas, you'd need a full wetsuit and a fair amount of grit to swim in the Atlantic come December. Rest assured, though, there's so much to do here, year-round. Even when it's colder there are hundreds of activities besides lying on the beach. (Perhaps golf, golf, and more golf, if that's your thing?).
Spring and autumn are both quieter, as school is in session, and are blissfully free of humidity. No matter the ambient temperature, many accommodations offer hot tubs or heated pools.
Summertime is the Florida you expect - sunny scorchers! The Atlantic heats up a little, too. If coming at this time of year, you'll want to be sensible when sightseeing or spending time outside - all the clever stuff about drinking enough water, sunscreen, etcetera.