The Sun's Birthday and the Contest Over 25 December

The Dies Natalis Solis Invicti, the Birthday of the Unconquered Sun, is one of the most argued-over dates in the history of religion. The sun god Sol Invictus was raised to the front rank of Roman state cult by the emperor Aurelian, who dedicated a lavish temple to him in 274 and instituted games in his honour, and a solar birthday near the winter solstice, when daylight visibly returns, suited such a god. Because the Christian feast of the Nativity settled on the same 25 December, two explanations have long competed.

The older History-of-Religions view, associated with Hermann Usener, holds that the Church deliberately chose the day to displace or absorb the popular solar feast. The rival computation hypothesis, argued by Thomas Talley and others, holds that Christians reached 25 December independently, by reckoning from an assumed date of the conception or Passion, so that the date owes nothing to Sol. Modern scholarship treats the question as genuinely open, and it is a standard caution against assuming that any shared date proves borrowing.