I noted you also broadened your claim to the possibility that the Winter Solstice could also have been a birthday for the Roman 1st century deity Mithras.
I could be wrong, but I’ve not seen that theory raised before. What are you basing it on?
The Romans observed the Winter Solstice, of course, just as most other cultures do, but it held no particular religious significance for them. It was an astronomical event that was useful in time-keeping, calendar-keeping, and agriculture.
The Romans, like most ancient peoples (including the Christian converts), were aware that the date varied from year to year but was not usually on December 25 (the inadequacies of the Imperial Roman Julian calendar system made it even more difficult). It was surprisingly not linked by the Romans to worship of a solar deity, and was not “owned” by the pagans any more than by the Christians, who certainly did recognize its symbolic nature, as St. Augustine noted early on.
In a Christmas Day sermon, St. Augustine proclaimed the idea, common among early Christians, that the date of December 25 was chosen by Jesus as His date to be born for its symbolic astronomical significance as well as to supplant (not assume or “baptize”) demonic pagan celebrations:
*"Our Lord Jesus Christ, who was with the Father before he was born of his mother, chose not only the mother of whom he would be born, but also the day on which he would be born. Fallible human beings often choose days, one for planting, another for building, another for setting out, and sometimes even for marrying a wife. When people do this, the reason they do it is in order that something already born, so to say, may grow up or turn out well. Nobody, however, can choose the day on which actually to be born. But he was able to choose both, because he was able to create both. Nor did he choose the day in the same way that people do, who vainly hang the fates of individuals on the dispositions of the stars. I mean, in this case the one who was born was not made lucky by the day, but he gave good luck to the day on which he was graciously pleased to be born.
"Because even the day of his birth contains the mystery of his light. That, you see, is what the apostle says: The night is far advanced, while the day has drawn near; let us throw off the works of darkness and put on the armor of light; let us walk decently as in the day (Rom 13:12-13). Let us recognize the day, and let us be the day. We were night, you see, when we were living as unbelievers. And this unbelief, which had covered the whole world as a kind of night, was to be diminished by the growth of faith; that’s why, on the day we celebrate the birth of our Lord Jesus Christ, the night begins to be encroached upon, and the day to grow longer.
“So, brothers and sisters, let us keep this day as a festival; not, like the unbelievers, because of that sun up there in the sky, but because of the one who made that sun. That which was the Word, you see, became flesh, in order to be able for our sakes to be under the sun. Under the sun, indeed, in the flesh; but in divine greatness over the entire universe, in which he placed the sun. Now, though, he is also over that sun even in the flesh, the sun which people worship instead of God, because in their mental blindness they cannot see the true sun of justice.”
*
What’s interesting about Augustine’s Christmas-Day sermons is that when he wrote them, the solstice occurred December 21st. The Julian calendar fixed the solar year at precisely 365 ¼ days. We now know that the actual length of the solar year is 365 days, 5 hours, 48 minutes, 45 seconds, just under eleven minutes shorter than the Julian year.
This means that in the space of 131 years, the Julian year fell behind the solar year by one day, and the solstices and equinoxes were occurring a day before their civil date in the calendar.
By A.D. 325, the solstice and equinox anticipated their civil dates by four days. Hence, to establish the uniform observance of Easter (Pasche), which is determined by the vernal equinox, the Council of Nicaea fixed the ecclesiastical date of the spring equinox at March 21st to coincide with the actual astronomical event. But since the four points of the year stand in fixed relation to one another, the winter solstice had occured four days earlier, at December 21st .
This means that when Augustine preached his Christmas sermons, actual connection between December 25th and the winter solstice had ceased to exist as much as four hundred years earlier. Selden (Theanthropos) took this as evidence that belief in the December 25th birth of Christ must have originated very early on, in apostolic times, while the association of the solstice with December 25th was still popularly retained, long before Nicaea when the distance between these events was widely known and understood.
So, no textual attestation that the solstice was a religious celebration for the imperial Romans, but early corroboration that the Winter Solstice was recognized for its symbolic (or per Augustine, intentional and sacred) association with the Nativity. No Mithraic connection that i can see there.