Sol

" December 25 was commonly indicated as the date of the winter solstice,[e] with the first detectable lengthening of daylight hours. 

 The Philocalian calendar of AD 354, part VI, gives a festival of natalis invicti on 25 December. There is limited evidence that this festival was celebrated before the mid-4th century. 

 The same Philocalian calendar, part VIII, also mentions the birth of Jesus Christ, stating that the "Lord Jesus Christ was born eight days before the calends of January" (that is, on December 25). 

 Since the 12th century, there have been speculations that the near-solstice date of 25 December for Christmas was selected because it was the date of the festival of dies natalis solis invicti, but historians of late antiquity make no mention of this, and others speculate" 

https://en.wikipedia.org/wiki/Sol_(Roman_mythology) 

"Sol is the personification of the Sun and a god in ancient Roman religion. "