Beth,
Weather can vary from mid 50's up to high 70's.......we did Discovery Cove on Dec 1st and it was really hot, we also had some long trouser days!!
Good thing is that Christmas decorations go up IMMEDIATELY after Thanksgiving and make everywhere look very festive.
I'd recommend you DEFINITELY avoid Thanksgiving Day if you're out there...the parks are impossible ,everyone and his dog will be there !!
Otherwise, just about everything in the parks is a "walk-on" minimal crowds....we loved it.
By the way, Thanskgiving Day is the 4th Thursday in November.
Hope this helps