The Romans held a festival on December 25 called Dies Natalis Solis Invicti, "the birthday of the undefeated sun." The use of the title Sol Invictus allowed several solar deities to be worshipped collectively, including Elah-Gabal, a Syrian sun god; Sol, the god of Emperor Aurelian (AD 270–274); and Mithras, a soldiers' god of Persian origin. Emperor Elagabalus (218–222) introduced the festival, and it reached the height of its popularity under Aurelian, who promoted it as an empire-wide holiday.
December 25 was considered the day upon which the winter solstice, which the Romans called bruma, fell. (When Julius Caesar introduced the Julian Calendar in 45 BC, December 25 was approximately the date of the solstice. In modern times, the solstice falls on December 21 or 22.) It is the day the Sun proves itself to be "unconquered" and begins its movement toward the north on the horizon. The Sol Invictus festival has a "strong claim on the responsibility" for the date of Christmas, according to the Catholic Encyclopedia. Several early Christian writers connected the rebirth of the sun to the birth of Jesus[16] "O, how wonderfully acted Providence that on that day on which that Sun was born . . . Christ should be born", Cyprian wrote.