OP,
Another poster posted how Disney makes you aware of the special offerings, but I would also note that if you are traveling from one park to another on Christmas Day that you should consider the extra time it takes to move through the crowds and lines. We found it taking at least 2-3 times longer to get around...even to make it to a park exit, grab Disney transportation, process into the next park and make it to your restaurant. It took us around 60 minutes to go from mid-point on Main Street to Afternoon Tea at the GF on Christmas Day. I reserved Afternoon Tea for the family to get away from the crowds...it turned out to be a great choice, very peaceful and turned out to be the best experience of the day...even for two teenage daughters.