The god behind the day is Sol Invictus, the Unconquered Sun, a solar deity raised to the very front rank of Roman state religion by the emperor Aurelian. Fresh from restoring a fractured empire, Aurelian in 274 dedicated a magnificent temple to Sol in Rome, endowed a new college of priests (pontifices Solis) to serve him, and instituted games in his honour to be held every four years. Sun-worship suited the age: the sun could be seen as the single visible power standing above and behind the many gods, a fitting patron for an emperor who wished to present his rule as universal and divinely upheld. Aurelian's coinage proclaims SOL INVICTUS and Sol, lord of the Roman empire, and the cult he established remained prominent through the following decades, its solar theology feeding into the religious atmosphere in which the emperor Constantine grew up.
The relationship between this cult and the Christian feast of Christmas on 25 December has been argued over by historians for more than a century. The older thesis, associated with the philologist Hermann Usener, held that the church fixed Christmas on the date of the Sol Invictus festival to supplant a popular pagan holiday. More recent scholarship, including work by Thomas Talley, has revived an alternative calculation hypothesis, that some Christian communities arrived at 25 December independently by reckoning nine months from a 25 March date already associated with Christ's conception, and has noted that the earliest firm attestation of 25 December as Sol Invictus's own festival date comes only from the mid-fourth-century Chronography of 354, roughly the same period as the earliest clear evidence for Christmas itself, leaving the question of which, if either, borrowed from the other genuinely unsettled among specialists.