Below is an informative series of chapters which describes biblical and historical evidence which points to December 25th as the accurate date of Christ's birth. After the links is the summary of the writing.
December 25th as the Actual Date of the Birth of Christ
In recent years one hears more and more frequently the view, that because the exact date of the birth of Christ is unknown to us, the Church placed Christmas on the alleged day of the pagan celebration of the "Invincible Sun" (the "god" Sol Invictus), as a symbolic gesture, to mean that Christ is the Sun of Righteousness, and in order to replace the pagan feast by overlapping it.
To the contrary, historical studies have shown that "the separation of the feast of Christmas from the feast of Theophany, as well as the choice of December 25th as a day of celebration, were made for pragmatic ecclesiastical reasons and not for reasons of copying or competing against a pagan model." Historical sources and records regarding this issue prove that the ecclesiastical establishment of the feast of Christmas for December 25th (which was celebrated for the first time in Rome in 336 A.D.) preceded that of Sol Invictus, which was a celebration established by Emperor Julian the Apostate in around 362 A.D. for December 25th, which was to replace the older feast of the "Birth of the Invincible" (Natal Invicti), which was not the sun. This move by Julian the Apostate in establishing solar feasts, was most likely an attempt to replace the Christmas feast with his own feast of the sun, and not the other way around. To establish this fact, we refer to the interpretation of Saint John Chrysostom, who testifies that the Church of Rome knew through its archives, as the capital of the Roman Empire when Christ was born, the exact date of the census of Caesar Augustus, when Christ was born (Lk. 2:1-7). Chrysostom also goes into detail concerning the biblical events from the time the birth of John the Forerunner was announced to his father Zechariah until the birth of Christ.
In this brief study, we will describe exactly how the date for the birth of Christ was determined, with the guidance of Saint John Chrysostom, "the meadow of the words of inspired Scripture."...
Part I
Part II
Part III
Part IV
Summary of the Above Biblical Determinations
Based on the above hermeneutical calculations of St. John Chrysostom, it has been demonstrated that Zechariah, the father of John the Forerunner, according to the first chapter of the Gospel of Luke, offered incense in the Holy of Holies according to the prescribed ritual for the feast of Atonement, as a High Priest. In detail of this ritual, he entered the Holy of Holies alone, without being visible to the people, and he offered incense on the golden altar of incense, which is in between the Holy Place and the Most Holy Place (Holy of the Holies). Therefore this could not have been a daily offering of incense in the Holy Place, like all the priests would do on a daily basis, rather all the relevant facts point to the feast of Atonement, which was between September and October, for it was on this day only that the High Priest would be alone in the Sanctuary. Immediately after those days of Atonement and Tabernacles and upon the return of Zechariah to his home, Elizabeth conceived and became pregnant (cf. Lk. 1:24). Six months later, in March, the Son and Word of God was conceived by the Theotokos and the Holy Spirit (cf. Lk. 1:26). From this is determined nine months when Christ was born, a total of fifteen months since John the Forerunner was conceived, which falls in the end of December, in agreement with Church tradition.
The Testimony of the Church of Rome
In the same homily of St. John Chrysostom, "On the Day of the Birth of Our Savior Christ", he also invokes the testimony of the Church of Rome which celebrated the Nativity of Christ on December 25th, and this is why the homily bears the subtitle: "Which was then still uncertain, but a few years ago made known and proclaimed on the part of ones who came from the West."
Concerning this testimony, Chrysostom says: "It is clear that He was born during the first census. And it is possible for the one who desires to know exactly to read the original codices publicly stored at Rome and learn the time of the census. So what, someone says, is this to us - who are neither there nor present? But listen, and do not be unbelieving, because we have received the day from those who know these things accurately and who dwell in that city. For the ones living there, having observed it from the beginning and from ancient tradition, now have themselves transmitted the knowledge of it to us."35 The Saint goes on to explain how divine Providence caused the soul of Caesar Augustus to initiate the first census of the ecumene in order to fulfill the prophecies for the birth of Christ in Bethlehem, even though the Theotokos lived in Nazareth, and thus the power of God is revealed: "Did you notice, beloved, the economy of God, the way He manages His (purposes) through unbelievers as well as believers, that strangers to piety might learn His might and power."36
The Orthodox Acceptance of Hostile Interpretations
Despite the above evidence, typical is the opinion of the preeminent contemporary Greek liturgist John Fountoulis, who writes: "With the epiphany of false gods and emperors, the Christian Church opposed this with the epiphany of the true God and King Christ, the true theophany. Also the worship of the sun, which conquers during the winter solstice the darkness of night, was opposed with the worship of the true sun, Christ, who rose, as the prophet Isaiah said, in a world sitting in darkness and shadows." He speaks of how the feasts of Christmas and Theophany originated to replace feasts of the sun and the celebration of the winter solstice. It was by Christians replacing these winter pagan feasts, that the date for the Presentation of Christ forty days after His Nativity and the date for the Annunciation nine months before the Nativity, and even the conception of St. John the Forerunner in September originated. Thus, everything in our immoveable calendar for Professor Fountoulis begins with "Sol Invictis" during the winter solstice of December 21-22.
To the contrary, such misconceptions ignore historical facts and the biblical and patristic tradition to explain the origins of our liturgical feasts. That such a revered professor and liturgist would hold such opinions reveals that even Orthodox are under an academic/theological captivity that directly opposes Orthodox tradition, whether we are ignorant of the facts or not.
Epilogue: The Role of Divine Providence
To conclude, the feast of Christmas was not determined by Christian leaders to replace the worship of the invincible sun, but as St. John Chrysostom writes, it was according to the divine economy, divine Providence, that the God-man was born during the census of Caesar Augustus, as we previously mentioned, and that the Conception of the Forerunner that opens the Gospel of Luke, which led to creation being refashioned through the Incarnation, should fall in September, when the world is said to have been created and the new year began according to Hebrew calculations. For the symbolic significance of the census taking placing during the days of the Incarnation, and that it was not a coincidence, the sacred hymnographer writes: "When it was time for Your coming to earth, the first imperial taxation was held, but You also took a census, O Lord, recording the names of all men who believed in Your birth. You used the decree of Caesar for Your own end: to make manifest Your timeless and eternal Kingdom. Therefore we pay You our taxes, not with golden coins, but with the riches of Orthodox theology, O God and Savior of our souls."38 It was thus according to the economy of the Holy Trinity, and not a "coincidence" that these sacred events took place, nor are they merely a replacement of pagan feasts.