Ever since I was little, I learned that Christmas was placed on December 25th because that was the Roman pagan holiday for the “Birth of the Unconquered Son.” The way i learned it, the Catholic Church adopted that day to represent the birth of Christ in an effort to convert pagans.
As time has gone on, it seems that this is not the case. Mark Shea teaches us today that it is actually the reverse. The pagan Roman Emperor Aurelian adopted the Christian date of Christ’s birtth to try to co-opt the growing Christian religion.
[T]he definitive “Handbook of Biblical Chronology” by professor Jack Finegan (Hendrickson, 1998 revised edition) cites an important reference in the “Chronicle” written by Hippolytus of Rome three decades before Aurelian launched his festival. Hippolytus said Jesus’ birth “took place eight days before the kalends of January,” that is, Dec. 25.
Tighe said there’s evidence that as early as the second and third centuries, Christians sought to fix the birth date to help determine the time of Jesus’ death and resurrection for the liturgical calendar—long before Christmas also became a festival.