One of the most popular theories and commonly taught explanations for why Christmas is on Dec. 25th is because the early church placed Christian holidays at times of Roman celebration to co-opt the local pagan festivals.
Christians placed Christmas on Dec. 25th to co-opt Saturnalia, the mid-winter festival, or possibly the Festival of the Unconquered Sun -- Sol Invictus. The theory went that Christians could get the heathen to convert by co-opting their own holidays.
There is one problem -- it sounds more convincing than it is. These theories did not start growing until the 12th century and only became popular once comparative religion became trendy after the 18th century. Going back to the earliest Christian church finds evidence that Christmas, though not initially celebrated, had starting being commemorated well before the Feast of the Unconquered Sun's creation for entirely Christian reasons.
In Egypt, less than 300 years after Christ's death, some Christians celebrated his birth in the spring. As the Biblical Archeology Society has noted, the earliest references to Christmas come at about 200 A.D., at a time Christians were not incorporating other religious traditions into their own. By 300 A.D., many Christians were celebrating his birth around Dec. 25th. Within 100 years, Christmas was on the calendar record. Christians looked to December because the early church was far more interested in Jesus's death. His death and resurrection is what matters to the Gospel, and that was the date the early church focused on.
"Around 200 A.D., Tertullian of Carthage reported the calculation that the 14th of Nisan in the year Jesus died was the equivalent to March 25 in the Roman calendar," said Andrew McGowan last year at the Biblical Archaeology Society. That would be the day of Crucifixion. The math from there is rather simple. Nine months later would be Dec. 25. Early church history held as fact that the prophets and martyrs of the church were conceived on the day they died. So if Christ died on March 25, it was also the anniversary of his conception.