Professor Joseph Kelly, Saturnalia, and the Inception of Christmas

Joseph Kelly, who has written a book on the origins of Christimas, also states:

"Many Romans venerated the sun, whose birthday was Dec. 25, or a virility god named Mithra with the same birthday. Also, the Romans observed a raucous celebration called Saturnalia Dec. 17-23. Thus, Dec. 25 offered a date with a good theological basis that also would counter several pagan holidays.

Although we don't know the final steps, in 336 the church at Rome officially observed the 'birth day of Christ' Dec. 25. This tradition spread."

See https://fosterheologicalreflections.blogspot.com/2014/12/joseph-kelly-true-meaning-of-christmas.html