Why Dec. 25? Here's one answer. [via] -- Read the article and check out references to Finegan's Handbook of Biblical Chronology and St. John Chrysostom as mentioned. Here's an excerpt:
So how did it become "common knowledge" that Christmas is really just a
warmed-over pagan festival? It happened through a series of ironies capped by
yet another example of pseudo-knowledge
The first irony is the reaction of the Christians of the late Roman Empire to Aurelian's attempt to co-opt Christmas and make it a pagan day of celebration. Instead of fighting with Sun-worshipers who were trying to rip off their feast, early Christians simply "re-appropriate[d] the pagan 'Birth of the Unconquered Sun' to refer, on the occasion of the birth of Christ, to the rising of the 'Sun of Salvation' or the 'Sun of Justice.'" Mark that, because we shall return to it.
Update I had to run to a meeting and didn't tweak this as much as I wanted and came back in for a bit. These misunderstandings give rise to some of the uneducated reasons for not celebrating Christmas--ever by Christians. Let's put our Information Literacy skills to work and check primary resource documents in Early Church History. (not that Finegan is primary.)