What best known pagan festival inspired many of todays traditional Christmas and Yuletide celebrations?
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.
