By far, the greatest myth that has evolved around the Christmas holiday is that it is a pagan festival with no bearing to the early church. Dr. William Tighe, a professor of history at Muhlenberg College in Allentown, writes a persuasive piece that attacks this idea:
I am speaking about all those people, pagans and "fundamentalists" alike who agree that "Christmas isn't Christian". It is all based on the idea, first invented in the late 17th Century, that the celebration of Christ's nativity on December 25th, was an attempt to "Christianize" a pagan festival.
This "tale" has been all-but-completely exploded by Early Church historians and liturgical scholars over the past 40 years. It now seems likely that the Roman Emperor Aurelian (d. 275) invented and established the "Feast of the Unconquered Sun" on December 25th to provide a government-sponsored pagan festival on a day of significance to Christians in Rome. It is true that the first evidence we have for a liturgical celebration of Christ's birth comes from ca. 338 AD, but the Latin Christian writer Tertullian gave December 25th as the date for Christ's birth writing around 220 AD, a half-century before the Emperor Aurelian established his new pagan festival on December 25th, and that fact alone ought to give the "Christmas is pagan" crowd pause.
(Lifted from Mere Comments last year.)
The best book on the subject is The Origins of the Liturgical Year, by Thomas J. Talley, an Episcopalian clergyman and retired professor at General Theological Seminary in New York. (The book is still in print and available from The Liturgical Press.) Talley's argument is essentially this: that in the Second Century there is evidence that Christian thinkers in both the Greek East and Latin West of the Roman Empire wanted to establish the date on which Christ died, but they went about it in different ways.
Greek Christians wanted to "translate" into their own solar calendar the date 14 Nisan in the Jewish lunar calendar, on which date Christ died (on the Eve of Passover) according to John's gospel. They simply chose the date 14 Artemision of the Greek calendar, both Nisan and Artemision being the months in their respective calendars in which the Spring equinox falls. When the Greek calendar was superseded by the Roman calendar around AD 300, 14 Artemision became 6 April.
Latin Christian thinkers wanted, rather, to establish the historical date on which Christ died, and came up with March 25, 29 AD. This is surely wrong; it can, in fact, have happened only in 30 or 33 AD, but that is what they thought. Next one has to consider a common Jewish belief at the time that all the great prophets died or were killed on the same day as that of their birth or conception, Talley continues. If Christ was conceived on the same calendar date as his supposed death, then one adds 9 months to 6 April and gets 6 January, which is Epiphany, or 9 months to 25 March, and gets 25 December, which is Christmas.
And in fact, in the Greek East, down till around 385 AD when they adopted it from the West, the Eastern Christians did not celebrate Christmas at all, only Epiphany, as Christ's manifestation in the world -- at his birth in Bethlehem, to the Three Magi, and at his Baptism in the Jordan, all in one feast. (The Armenian Church never has adopted Christmas, and still celebrates "ancient Epiphany" alone on January 7th.) St. John Chrysostom in a sermon preached in his native Antioch around 390 AD speaks of the "recent adoption" of the December 25th feast. And in the Latin West, December 25th alone was celebrated for many centuries, and when the West did adopt the January 6th Epiphany from the East it never made much of it.
The point of all this is that the idea that Christmas (December 25th) as a Christian festival was an adoption of a "pagan" festival is almost certainly false. It had a certain degree plausibility at one time, but is now completely indefensible. Ironically, the idea was invented in the late 17th Century by a couple of hyper-calvinistic scholars who wished to "prove" that Catholicism was really a pagan religion with a superficial Christian veneer, and in their notions about the origins of Christmas they thought that they had struck gold.