Other feasts than these [Easter, Pentecost] have an altogether different significance. Christmas is of comparatively recent origin. It does not, strictly speaking, commemorate a mystery, but a particularly unique natale, a birthday, and one falling on a specially suggestive day. Augustine knows that it replaces the birthday of Sol Invictus, although the pagan festival was only celebrated at a few places and was originally a particularity of the Roman city calendar. This fact, however, seems to have been widely known in Africa, since Augustine says in a Christmas sermon, “Let us celebrate this day as a feast not for the sake of this sun, which is beheld by unbelievers as much as by ourselves, but for the sake of him who created the sun.” [Sermo 190] He also believes, however, that there is a reliable tradition which gives December 25 as the actual date of the birth of our Lord. According to this same tradition the birth of John the Baptist took place six months before the eighth day before the calends of January, that is, on the eighth day before the calends of July—which means, unfortunately, that it fell on the 24th, and not on the 25th, of June.
Broadly speaking, however, the feast of St. John coincides with the summer solstice and Christmas with the winter solstice. This fact does not particularly disturb the good bishop. He merely points out that as Christmas the days begin to grow longer and on the feast of St. John to grow shorter, a symbol to show that the one had of necessity to become greater, while the other had to grow less.
F. van der Meer, Augustine the Bishop (292-293)