The early Christian communities, as recorded by Clement of Alexandria, began celebrating Jesus’ birth sometime around 200 AD.
Though the date varied by community, the two dates which became most common were Dec 25 and Jan 6 (the Feast of the Epiphany, which became in the Eastern Churches a more prominent feast than Christmas); in fact, he notes the Basilidians celebrating the Epiphany on January 6. It should also be understood that the celebration of the birth of saints etc. was developed over time, and was not initially practised; hence, the slow development of Christmas.
The pagan Romans did not celebrate the feast of the Sun God on Dec 25 until 274 AD, when Emperor Aurelian dedicated a temple to Sol Invictus (the Unconquered Sun) and named him the premier deity of the empire.
Prior to this, the Sun God (Mithras) had only been a minor Roman deity worshipped by soldiers, whose feast was celebrated on the solstice.
The early Christian tradition that Jesus was born on Dec 25 stemmed from the Jewish belief that prophets died on the anniversary of their conception.
Since Jesus was crucified on Nisan 14 (equivalent to March 25), according to the Gospel of St. John, this implied that He had been conceived on March 25 (hence the Feast of the Annunciation) and born on Dec 25.
As to the uniformity of the date, one must consider that Christians could not freely meet until the time of Emperor Constantine.
In fact, it is known that in Rome by 354 AD on the calendar for venerated martyrs is seen “natus Christus in Betleem Iudeæ”.
There is simply no evidence in any of the writings of the early Church Fathers that Christmas was assigned to Dec 25 to subvert pagan worship to the Sun God.
In fact, they demonstrate a disdain of pagan festivities and discourage associating Jesus Christ with the Sun God.
The Dec 25 feast of Sol Invictus appears to have been the Romans’ attempt to paganise a Christian festival, rather than the other way around. n
Louis F Figueroa