That would actually be the Pagan/Wiccan celebration of Yule... The birth of God. This is why Christians chose to celebrate the birth of their god at this time of year. Historically, Jesus was NOT born in December. However, as the world around them celebrated the birth of God during the Pagan festival of Yule, it would be easier to gain acceptance by adopting this holiday as their own.
The best answer is probably none. The "pagan" influence on Christmas is overstated and often wrong. Yule is a pagan or non-Christian celebration so that part answers itself. Traditions like Christmas trees for example were not based on any pagan religion.