December 25, 274
Emperor Aurelian makes Sol Invictus to an official cult, it becomes the state religion of Rome.
Repousse silver disc of Sol Invictus
Sol Invictus ("Invincible Sun") was the official sun god of the later Roman empire. In 274 Aurelian made 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 until Constantine. 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. A festival on 25 December is thought to be responsible for the date of Christmas.