Sol Invictus

Sol Invictus ("Unconquered Sun") was the official sun god of the later Roman Empire and a patron of soldiers. In 274 the Roman emperor Aurelianmade it an official cult alongside the traditional Roman cults. Scholars disagree whether the new deity was a refoundation of the ancient Latin cult of Sol, a revival of the cult of Elagabalus^] or completely new. The god was favored by emperors after Aurelian and appeared on their coins untilConstantine. The last inscription referring to Sol Invictus dates to 387 AD and there were enough devotees in the 5th century that Augustine found it necessary to preach against them.

It is commonly claimed that the date of 25 December for Christmas was selected in order to correspond with the Roman festival of Dies Natalis Solis Invicti, or "Birthday of the Unconquered Sun", but this view is challenged.

Source: https://en.wikipedia.org/wiki/Sol_Invictus