The contention that December 25th was an especially popular festival for Sol in late antiquity is equally unfounded, as is as the notion that this festival was established by Aurelian when he supposedly instituted a new cult of the sun. Aurelian did of course build the sun a magnificent new temple and he raised the priests of Sol to the level of pontifices. A new festival on December 25th would not have been out-of-place in this context, but it must be stressed, pace Usener, that there is no evidence that Aurelian instituted a celebration of Sol on that day. A feast day for Sol on December 25th is not mentioned until eighty years later, in the Calendar of 354 and, subsequently, in 362 by Julian in his Oration to King Helios. In short, while the winter solstice on or around the 25th of December was well established in the Roman imperial calendar, there is no evidence that a religious celebration of Sol on that day antedated the celebration of Christmas, and none that indicates that Aurelian had a hand in its institution. One might think that celebrating the sun on the winter solstice is so self-evident that we need hardly doubt that such a festival had a long tradition, but what evidence we have actually belies that notion. The traditional feast days of Sol, as recorded in the early imperial fasti, were August 8th and/or August 9th, possibly August 28th, and December 11th. These are all dates that are unrelated to any important celestial alignment of Sol, such as the solstices and equinoxes. Of these traditional dates only August 28 is mentioned in the Calendar of 354, along with December 25th, and multi-day games from October 19th to October 22nd, the latter being the most important, judging by the unparalleled 36 chariot races with which it was celebrated.
...To summarize, the Calendar of 354 mentions the following three festivals of Sol:
- August 28: Sol and Luna; 24 chariot races;
- October 19-22: Ludi Solis, 36 chariot races;
- December 25: Natalis Invicti, 30 chariot races.
The standard norm for Roman festivals at this time was 24 chariot races. Of the 63 race-days listed in the Calendar of 354, 59 had 24 races, the only exceptions being February 25th and June 1st, when only 12 races were held, December 25th with 30 races, and October 22nd with 36 races. The 36 chariot races of October 22 thus represent the highest number of the year, which further suggests that this was not an annual, but a rarer, quadrennial celebration. This means that in the early fourth century, when Christmas was established by the church on December 25th, anyone surveying the calendar of festivities in honour of Sol would identify the period from October 19th to October 22nd as far more important than December 25th, and the festival of August 28th as far older. If the aim was to “neutralize” the cult of Sol by “taking over” its major festival, December 25th seems the least likely choice. It is true, of course, that December 25th, the natalis invicti, was the traditional date for the winter solstice and as such the most logical of the three dates to serve as birthday of Christ, if that was the way the “selection process” went. But this leads us to a different consideration. As we have seen, none of the traditional religious feast days for Sol were connected in any way with a specific astronomical date, such as one of the solstices or equinoxes. Yet we know from the abundance of iconographic and other evidence that astronomical dates and phenomena such as the solstices and equinoxes played an important role in Roman society from at least the 1st c. BC onwards. One need but think of the numerous depictions of seasons, zodiacs, calendars and planetary gods representing the days of the week, already quite prominent in Pompeian art in the early first century AD, and popular throughout the Roman imperial period. The parapegmata (peg-calendars) discovered throughout the empire further attest to the pervasive influence of such concepts as the seven-day planetary week. This evidence is suggestive, because it indicates that there was great interest in the astronomical, astrological and calendrical aspects of the sun, sol, as heavenly body, without these being central to the cult calendar of the Roman Sun god, Sol. All this makes it clear that the anomalies surrounding the natalis invicti (birthday of the invincible one) on December 25th are quite striking.
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