I haven't been on Hilton Head Island on Christmas Day, but I have spent a week in early December there. We had one day that was sunny and 70 degrees. The other days were in the 50s and 60s, and we had quite a bit of rain. There were a lot of nice Christmas decorations, but the island was very quiet. A little too quiet for my taste. Many of the restaurants were closed, and there were not many activities going on. I knew it would be off season, but I was surprised at just how quiet it was.
Of course, the island was still beautiful and we had a good time. But I have had a better time at other times of the year. On a positive note, Savannah has awesome Christmas decorations and it is well worth a visit during the holidays.
Steve