February 27, 380, the emperor Theodosius abolished the religion and festival of the Sun Invictus by the famous edict of Thessalonica. Until then the Romans worshiped the Sun god during the Republic, and with greater prominence in the Imperial era, since the gradual entry of different religions of salvation of monotheistic tendency, such as Christianity and Mithrism, was giving greater weight to the power of the star king, the one who conquered the darkness ...
Emperor Aurelian made it official on December 25 as the day of the birth of the Undefeated Sun (the main divinity of the Roman pantheon at that time) back in the year 274 and later Juliano the Apostate would declare Helios (Greek equivalent) as the only divinity, and would try to prevent the spread of growing Christianity.
Finally, during a long process of syncretism, both the Festival of the Birth of the Invincible Sun "Dies Natalis Solis Invicti" held after the Winter Solstice, when daylight increased, as well as many other acts and celebrations were gradually reconverted. Paganism would come to an end, a new stage would begin with a victorious sun reborn in the arms of a Galilean.