Sol Invictus, the Unconquered Sun, was a Syrian god established as a supreme deity by the emperor Aurelian in AD 273. According to Lesley Adkins and Roy Adkins in their Handbook to Life in Ancient Rome: "It has been suggested that the cult of the sun influenced the east-west orientation of burials, so that the dead arose to face the rising sun on the day of resurrection. It is more certain that the sun's birthday (25 December: the midwinter solstice in the Julian calendar) could not be suppressed by Christianity, and so the festival was made out to be the birthday of Christ instead."

The day was notable in the Roman astronomical calendar as the day of the winter solstice (bruma), but became more loosely used for "midwinter." In Byzantine days, the Brumalia was celebrated as a festival, and in Byzantium itself it lasted until the tenth century at least.