The actual date of Christ’s birth is unknown but by the early fourth century the church of  Rome designated December 25 for its celebration. This date may have been chosen to counter the pagan feast established by Emperor Aurelian in 274 to honor the Syrian sun god.  Christians could now celebrate Christ, the true “sun of justice.” 

Another theory says that the origin of Christmas was established during a time when the cult of the sun was especially strong in Rome and Christ-as-sun was deeply rooted in Christian consciousness. Since no evidence of a date could be found through Scripture, theologians turned to the equinoxes and solstices. Popular opinion held that John the Baptist was conceived on the autumn equinox and born on the summer solstice on June 24. Since the gospel of Luke tells us that Jesus was conceived six months after John, that would put his conception at the spring equinox on March 25 and his birth nine months later. Whatever the reason, December 25 was a convenient day to celebrate the birth of Christ and oppose the pagan feast.