We've been to WDW in early to mid-December for several years. Generally speaking, the weather is quite pleasant - comfortably warm during the day and only mildly chilly in the evening. But, as previous posters have noted, there are exceptions. I find that the best thing to do is check the weather a couple days before you leave for your week. It will give you a pretty accurate idea of the temperature trends for that week, which don't tend to change as much as something like precipitation predictions. If it's predicting unusually chilly weather, you'll know to bring more warm clothing and layers. If the outlook is for comfortable temps, you can go easy on the fleeces and sweatshirts and pack more shorts and tees.