Hi Kathy!
What a great time of year you have chosen to visit Walt Disney World!! I personally feel that visiting betweeen Thanksgiving and Christmas is the best opportunity to AVOID crowds. Even though park attendance is unpredictable, historically, this time of year is very manageable in terms of crowds, and as a result, lines and wait times.
Even better than reduced wait times during December is the fact that all of the parks and resorts are totally decked out for the holidays and it is really quite spectacular!
One thing to remember is that due to Mickey's Very Merry Christmas Party, the Magic Kingdom closes at 7PM on select nights. Yet other nights, particularly weekends, the Magic Kingdom, and other parks, may be open until after midnight.
Check back at
www.waltdisneyworld.com in late June or so to see actual park hours and entertainment schedules.
I hope this helps, and I wish you a Magical (& merry) vacation in December!! :)
Chris