The time has come to summarise some of the findings of the dozen or so posts on questions related to whether Christmas, on 25 December, was on the winter solstice in antiquity. I think we can say with certainty that it was thought to be on 25 December, or at least when the solstice was marked. I will return to this last point after reviewing the evidence associating 25 December with the solstice.
Dicta bruma, quod brevissimus tunc dies est; solstitium, quod sol eo die sistere videbatur, quo ad nos versum proximus est.
Bruma is so named, because then the day is brevissimus ’shortest’: the solstitium, because on that day the sol ’sun’ seems sistere ‘to halt,’ on which it is nearest to us.
In the 1st century AD, Ovid also tells us in his Fasti 1.161 that bruma is the new sun:
bruma novi prima est veterisque novissima solis
Midwinter is the beginning of the new sun and the end of the old one
In the 1st century AD, Pliny the Elder tells us in his Natural History 18.221, discussing the solstices and equinox that the bruma — which he still understands as the winter solstice — begins on 25 December:
… omnesque eae differentiae fiunt in octavis partibus signorum, bruma capricorni a. d. VIII kal. Ian. fere, aequinoctium vernum arietis, solstitium cancri, alterumque aequinoctium librae, …
the bruma begins at the eighth degree of Capricorn, the eighth day before the calends of January, …
In the 3rd century we get our first Christian connection of the birth of Christ with the sun. Cyprian, in De pascha Computus, 19 writes:
O quam præclare providentia ut illo die quo natus est Sol . . . nasceretur Christus.
O, how wonderfully acted Providence that on that day on which the Sun was born . . . Christ should be born.
In the 4th century, Servius tells us in his Commentary on the Aeneid 7. 720 that the “new sun” is 25 December. Commenting on the words of Vergil (underlined):
 vel cum sole novo prima aestatis parte: nam proprie sol novus est VIII. Kal. ian.; sed tunc non sunt aristae, quas ab ariditate dictas esse constat.
Or when the new sun in the first part of the year; for properly the new sun is the 8th day before the kalends of January; but at that time there are no harvests, which ab ariditate dictas esse constat.
In 354 AD the Chronography of 354 displays on 25 December, the VIII kal. Ian., “Natalis Invicti”, presumably the natalis of Sol Invictus. This may be either the birth of the unconquered sun, or the anniversary of the dedication of the temple.
In the fourth century Gregory of Nyssa comments in his Sermon on the nativity of the Saviour:
And again let us resume it: “This is the day which the Lord has made, let us rejoice and be glad in it,” – (the day) on which the darkness begins to decrease, and the lengths of night are diminished by the increase of the sun’s rays.
At the end of the 4th century, or perhaps later, ps.Chrysostom preaches on the solstice and the equinox. The sermon de Solst. Et Æquin. has never been translated, but the following excerpt appears in the Catholic Encycloped, giving a reference to the 1588 edition of “II, p. 118, ed. 1588”. I suspect in reality the material is in Migne! This also identifies the date with the sun, and here is clearly the birth of the new sun. It says:
Sed et dominus noster nascitur mense decembris . . . VIII Kal. Ian. . . . Sed et Invicti Natalem appelant. Quis utique tam invictus nisi dominus noster? . . . Vel quod dicant Solis esse natalem, ipse est Sol iustitiae.
But Our Lord, too, is born in the month of December . . . the eighth before the calends of January . . . But they call it the ‘Birthday of the Unconquered’. Who indeed is so unconquered as Our Lord. . .? Or, if they say that it is the birthday of the Sun, He is the Sun of Justice.
In 401 AD, on Christmas day, Augustine (PL 46, 996) preaches a sermon discussing pagan customs on the same day:
Stop these latest sacrileges, stop this craze for vanities and pointless games, stop these customs, which no longer take place in honour of demons but still follow the rites of demons … Yesterday, after vespers, the whole city was aflame with stinking fires; the entire sky was covered with smoke! If you make little of the matter of religion, think at least of the wrong that you do to the community. We know, brothers, that it is kids who have done this, but the parents must have let them sin.
In the 5th century Paulinus of Nola in Carmen 14, 13, links the birth of Christ at Christmas with the solstice (“days of bruma”), and the new sun explicitly:
ergo dies, tanto quae munere condidit alto Felicem caelo, sacris sollemnibus ista est, quae post solstitium, quo Christus corpore natus sole nouo gelidae mutauit tempora brumae atque salutiferum praestans mortalibus ortum procedente die se cum decrescere noctes iussit, ab hoc quae lux oritur uicesima nobis, sidereum meriti signat Felicis honorem.
13. So the day which bestowed so great a gift by setting Felix in the heights of heaven is the day of our yearly ritual. It comes after the solstice, the time when Christ was born in the flesh and transformed the cold winter season with a new sun, when He granted men His birth that brings salvation, and ordered the nights to shorten and the daylight to grow with Himself. The twentieth day that dawns on us after the solstice marks the heavenly glory which Felix merited.
Also in the 5th century, the calendar of Polemius Silvius has an entry for 25 December:
25 VIII natalis domini corporalis solstitium et initium hiberni
25 8 birthday of the Lord in the flesh solstice and beginning of winter
The VIII is the count down to the kalends of January.
It is pretty clear, then, that the date of 25 December was understood as being the winter solstice, and was marked as such at least in the fourth century onwards.
I was also interested in whether we could tell whether 25 December really was the astronomical solstice under the Julian calendar. This I was unable to determine. But it may not have been. The solstice moves, even under the Gregorian calendar, and only astronomers in antiquity would have been measuring it exactly. That the solstice had passed would become apparent to most people only a day or two later, perhaps. Some remarks by Julian the Apostate in 361 — over the Christmas period — in his Hymn to King Helios are interesting in this context, as they reflect this idea of deferral. The Kronia is of course the Greek for Saturnalia.
But our forefathers, from the time of the most divine king Numa, paid still greater reverence to the god Helios. They ignored the question of mere utility, I think, because they were naturally religious and endowed with unusual intelligence ; but they saw that he is the cause of all that is useful, and so they ordered the observance of the New Year to correspond with the present season; that is to say when King Helios returns to us again, and leaving the region furthest south and, rounding Capricorn as though it were a goal-post, advances from the south to the north to give us our share of the blessings of the year. And that our forefathers, because they comprehended this correctly, thus established the beginning of the year, one may perceive from the following. For it was not, I think, the time when the god turns, but the time when he becomes visible to all men, as he travels from south to north,that they appointed for the festival. For still unknown to them was the nicety of those laws which the Chaldaeans and Egyptians discovered, and which Hipparchus and Ptolemy perfected : but they judged simply by sense-perception, and were limited to what they could actually see. But the truth of these facts was recognised, as I said, by a later generation.
Before the beginning of the year, at the end of the month which is called after Kronos, we celebrate in honour of Helios the most splendid games, and we dedicate the festival to the Invincible Sun. And after this it is not lawful to perform any of the shows that belong to the last month, gloomy as they are, though necessary. But, in the cycle, immediately after the end of the Kronia follow the Heliaia. That festival may the ruling gods grant me to praise and to celebrate with sacrifice ! And above all the others may Helios himself, the King of the All, grant me this, even he who from eternity has proceeded from the generative substance of the Good: even he who is midmost of the midmost intellectual gods ; who fills them with continuity and endless beauty and superabundance of generative power and perfect reason, yea with all blessings at once, and independently of time !”
But the end result of all of this seems perfectly clear; in the 4-5th centuries, Christmas day was on the day of the winter solstice, as far as anyone knew, and Christ was born with the new sun, as the Sun of Justice, Sol Iustitiae.