Various opinions have been held about the way these dates were chosen. Occasionally it is suggested that December 25th is an adaptation of Jewish festival, but the 4C is too late for Jewish influence to be at all probable. In any case, the Jewish festival in question, the Rededication of the Temple by Judas Maccabaeus (Hanukkah), has quite a different meaning, lasts for eight days, and, through it begins on the 25th day of Chislev, Chislev is a lunar month corresponding only roughly to November or December.
The explanation most widespread today is quite different, namely, that December 25th and January 6th are derived from pagan sun-festivals. December 25th is a well-known date for the winter solstice, and, although sun-worship was not originally part of Roman religion, by the 3C it had become such, and a festival for the worship of the sun was established on December 25th by the emperor Aurelian in AD 274. January 6th, however, is only a very hypothetical day for the winter solstice, and no pagan festival on that day is recorded, except a festival of the goddess Core (Persephone) held at Alexandria, to celebrate her annual return from Hades; so the explanation is incomplete. One of the Greek festivals of Dionysus was in January (Lenaea, the “festival of the raving women”), but it was later in the month, and an orgy of this kind would be more likely to have given rise to a Christian fast than a Christian feast. The Western church may perhaps have reinterpreted the festival on December 25th as referring to Christ, the Sun of righteousness, so as to give the pagan observance an edifying new meaning, but what about the Eastern Church and January 6th?
Since January 6th can hardly have been the Christianization of a pagan festival, and was not a turning point in the astronomical year, it prompts a question whether the corresponding western date can have been merely that and no more. After all, December 25th as a date for Christ’s nativity is quite possibly older than the Christian or even the pagan festival on that date, since it occurs in Hippolytus’s Commentary on Daniel 4:23. The text of this passage is somewhat uncertain, it is true, and may be due to an early redactor rather than to Hippolytus himself. The other date for Christ’s nativity, however, can be traced back with greater certainty behind Hippolytus, to Clement of Alexandria, who before the year 200 dates Christ’s nativity on January 6th. This is over a century before any festival of the nativity on January 6th is recorded. Could Clement’s dating, then, be due to a historical tradition that the nativity took place at that time?
Browne’s and Bainton’s articles ought to be much more widely read than they are, for there is still today a strong tendency to assume that a midwinter date for the nativity is not even one of the earliest surviving traditions, and that this date must be due either to the Christianization of a pagan festival at that time of year, or to the contemporary speculation about the “appropriate” length for Christ’s life and its “necessary” alignment with the seasons. If, however, the traditional eastern day of January 6th was known in the church of Alexandria in the last decade of the 2C, it is as old as any of these speculations, and older than any evidence linking the nativity with the pagan festival on the winter solstice. Moreover, if it was known in Alexandria in the last decade of the 2C, it was probably also known there half a century earlier. For in the same passage of Clement, after speaking of the dates for the Lord’s birth, he says, “And the followers of Basiledes hold the day of his baptism as a festival…”
Basiledes likewise belonged to Alexandria, where he taught in the second quarter of the 2C, and though he was a heretic, he would have known the traditions of the Alexandrian church…Tertullian’s knowledge of January 6th as the date of Christ’s birth is confirmed by his apparent knowledge of it as the day of Christ’s baptism, for we have seen that anciently the date commemorated both events.
R. Beckwith, Calendar & Chronology, Jewish and Christian (Brill, 1996), 71-75.