Dies Natalis Solis Invicti

Deus Sol Invictus - invincible Sun God - was a title applied to at least 3 distinct deities, during the later Roman Empire.

 
Earlier cult of Sol Indiges was agrarian.

Romans held a festival on December 25,  Dies Natalis Solis Invicti - birthday of the invincible Sun. 

Emperor Elagabalus introduced the festival and it reached the height of popularity under Aurelian. He promoted it as an empire-wide holiday - a possible inspiration of Christmas. 

-  Wikipedia