Tunisia does not really celebrate Christmas as a country. However hotels may choose to adopt the theme for the benifit of their guests. The weather is warm (unlikely to be hot and could be mixed).
Popular resorts are:
Hammamet (north) very busy and built up
Skanes and Monastir (middle) A bit drab and Souless
Port el Kantaoui (middle) nicer feeling or attractive
Djerba (island) more western but relaxed and quieter
The hotels are generally a good standard, food average.
I would recommend visiting a few sights when there such as , Tunis (old town) El Djem (amphitheatre), Gafsa (old town and market), Douz (desert), Carthage (ancient city), Sidi Bou Said (village), Makthar (ruins), Dougga (roman ruins), Matmata (from Star wars)
You can easily book tours when you get there, the over night ones are best.
I would recomend a desert tour which includes el Djem, gafsa, matmata, douz.