Did Christians "christianize" the pagan feast of Sol Invictus (Birth of the Unconquered Sun) and make it Christmas on December 25th?
The short answer is ABSOLUTELY NOT, and it is refreshing to see at least a few articles on the internet disproving this long held myth perpetuated by Atheists, Pagans and Protestants.
Below are a few of my favorite articles which undoubtedly demonstrate that the Christian celebration of Christmas on December 25 came before the pagan festival of Sol Invictus rather than the other way around:
Calculating Christmas
How December 25 Became Christmas