Hi Katie,
My family visited Walt Disney World during the winter holidays last year, and it was truly magical. There are so many wonderful events around the winter holidays. From attending
Mickey's Very Merry Christmas Party to touring the resorts and their beautiful holiday decorations, it is a lovely time of year.
Since you are visiting for an extended period of time, you will likely see a shift in how busy the theme parks are. When you first arrive they should be relatively quiet, as you will still be quite far ahead of Christmas. The crowds will likely build toward Christmas and remain quite busy until December 29. Typically the week between Christmas and New Year's is extremely busy at Walt Disney World. I suggest that you plan the bulk of your theme park touring for early in your vacation. When I am looking for predictions of crowd levels at Walt Disney World, I like to visit the unofficial site touringplans.com. For a low price, you can have access to their predictions for crowd levels at Walt Disney World for up to a year.
As to the weather, while December is still very nice at Walt Disney World, it is of course one of the chillier months. The average high temperature is 73 degrees Fahrenheit, and the average low temperature is 52 degrees Fahrenheit. Disney does heat the pools at the resorts, so they may feel warm when you are in them; however, it is when you get out that you need to bundle up and get warm quickly! Also be sure to check park hours as you get closer to your vacation. Disney tends to close one water park at a time during the cooler months. Either Disney's Blizzard Beach Water Park or Disney's Typhoon Lagoon Water Park will remain open while the other is closed.
I am limited in the space that I have to answer, Katie. Please do write back if you have any further questions!