Was There Even a Festival on 25 December?

It is worth being candid about how thin the direct evidence for a Dies Natalis Solis Invicti actually is. The single explicit source is the Chronograph of 354 (the Philocalian Calendar), which against 25 December notes N(atalis) Invicti with thirty chariot-races, that is, games for the birthday of the Unconquered One. From this one late notice much has been built.

Some scholars take it to record an established solar feast that the Church then chose to overwrite; others point out that the same document, in another of its lists, is the earliest witness to the Christian celebration of the Nativity on 25 December, and question whether a major pagan solar festival on that exact day is really attested before the Christian one, or whether the two grew up side by side. The honest position is that a solar observance near the winter solstice fits Aurelian's cult well, but the precise history of 25 December as a Sol festival rests on slender ground and remains debated.