An exact date was attempted to be calculated for the Nativity of the Lord but it was deemed impossible (there was/is not enough information available to determine this). So originally, March 25, the first day of spring, was discussed as an appropriate day to celebrate the birth of Christ to coincide with the re-birth of the spring! However, other scholars noted that this would be a better day to place Jesus’ conception, as we believe that God becomes incarnate at the moment he is in Mary’s womb.
Therefore, if we add nine months to that date we get … December 25!
Secondarily, many Romans were sun worshipers. Many celebrated a kind of sun feast day on December 25, while others note a virility god named Mithra with the same birthday.
Lastly, the Romans observed a debaucherous time of year called Saturnalia December 17-23. Thus, December 25 offered a date with a good theological basis that also would counter several pagan holidays.
In 336, the Emperor Constantine officially named the “birth day of Christ” December 25.