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.