we used to go early december every year - some years it was swimming weather the entire time, other years it was quite cool.....
i have to find the picture of the kids on the boat to the magic kingdom, sitting in their double stroller, all bundled up in their snowsuits, mittens and balaclavas....it was COLD that year....
fortunately, since we were living in michigan at the time, we had our winter jackets/mittens with us, otherwise we would have all frozen to death...
however, most years the weather was mild, but the possibility of cold means you have to do as everyone has already said - bring layers so you'll be prepared...