We have done the week before Christmas twice and both times have had rain ruin some of our plans. Last year
was a whole day of crazy rain and tornado warnings. We have had much better luck in January and February.
It could be cold, mild, hot, or rainy. Or some combination of all of those in the same day. Typically it starts out chilly in the mornings and warms up midmorning and cools down in the evening. We haven't experienced a ton of rain in the past, but the possibility always exists. You have to pack for everything.
Pack so you can dress in layers! The weather at Christmas can be just about anything. Pack your swimsuit (one year it was 92 on Christmas Day. Pack gloves on morning it was 40 degrees, the monorail wasn't running. We had to ride the ferry boat from Ticket & Transportation Center - damp and freezing cold breeze.
There’s no telling what Florida weather will be between the end of October and early March. We’ve had everything from pool days to actual snow during Christmas while I’ve been living in Florida. Pack layers and expect the weather to change at the drop of a hat. You could have 30 degree weather on Monday and it’ll be 85 on Wednesday afternoon. Expect it to be fairly nice, maybe rainy, bring a jacket, definitely a pair of long pants.
A couple of years ago I was there for Christmas and the first few days were warm enough for me to wear shorts and a t-shirt around the parks. The next week and a half, however, there was a lot of rain and the high temps were upper 40s and low 50s. But then a couple of years before that the temps were closing in on 90 the whole time I was there.
Checking a long range weather forecast before you pack clothes is very helpful.