Aurelian dedicated the temple of Sol Invictus in 274 CE. The Chronography of 354, specifically its 336 calendar, marks December 25 as N INVICTI CM XXX, birthday of the Invincible, thirty races, and, in its martyrology section, natus Christus in Betleem Iudeae, the first record of Christmas on that date. Augustine's Christmas sermon warns his flock: celebrate not like the faithless, on account of the Sun, but because of Him who made the Sun. A fourth-century tract simply annexes it: who is as invincible as our Lord, who defeated death?
The standard story: Christians borrowed the pagans' solar festival. Steven Hijmans's revision: the evidence for a December 25 Sol festival before the 330s is thin, the calendar entry is anomalous, thirty races, not a multiple of twelve like Sol's other feasts, and the pagans may have built their December 25 in response to the Christian date, not the reverse. The alternative, the calculation hypothesis, derives Christmas internally: conception on March 25, the equinox, birth nine months later, rabbinic logic that great men die on their conception date.
The claim: the direction of borrowing hardly matters, December 25 was contested real estate because the winter solstice is the year's most potent symbolic slot, light's first gain, and every rising cult needed it; the calendar page is a bidding record, not a birth certificate. Verdict: Ovid called the solstice the first of the new sun before either party existed. The date was always going to be sacred. The only question the record leaves open is who fenced it first, and the fence-posts are now unreadable, which both sides should find instructive.
Did the Church steal the sun's birthday, or did the sun steal the Son's?