There are two main competing theories on why December 25 was selected as Christmas Day by the Western church. The dominant theory is that as Christianity extended its religious supremacy it co-opted existing pagan festivals and customs to ease the transition. The other hypothesis is that the Church calculated the date using some traditional assumptions. Neither theory provides a definitive explanation for the date selected.
The pagan festival that occurred on December 25 in the Roman world was called dies natalis solis invicti or birthday of the invincible sun. The Julian calendar was in use at this point in time so December 25 was the winter solstice. While solar worship has a long history in many cultures, this particular feast did not become prominent until the emperor Aurelian (ruled 270-275 CE). The empire was struggling in the third century. There were both external threats and internal divisions (over thirty emperors over fifty years). Historians theorize that Aurelian sought to unify the empire under a single religion and chose this particular sun god - Sol Invictus. A temple was dedicated to this god by Aurelian on December 25, 274. This is considered to be the high point for worship of this god. Only fifty years later, Constantine was providing official support to Christianity.
The theory is that the Christian Church selected December 25 as the birth day of Christ to take advantage of the already established festival - or at least to serve as a distraction. This would be similar to the harvest festivals that many American evangelical churches sponsor in order to compete against Halloween. There is not any direct evidence for this theory, but it is plausible given the number of pagan customs accepted by the church for its festivals (Christmas trees, Easter eggs, etc.).
Beyond the lack of primary sources, this theory has another weakness. Aurelian was murdered after ruling for only five years. The religion he encouraged did remain a player in the extremely diverse religious environment, but it is questionable whether its festival was established enough to attract the attention of the Church. Some have even suggested the festival date was chosen to compete with Christmas, but this has no evidence in its favor.
There is no evidence that early Christians celebrated the birthday of Jesus. There are a few mentions of attempts to determine the date of his birth, but they are not described as important by the sources. The earliest definitive mention of a Christmas celebration is in the mid-fourth century. (Christmas is an English term. It was called the Feast of the Nativity.) Historians think it began in Rome and then spread slowly throughout the empire. It was also not considered an important Christian festival at the time (Augustine did not classify it as such, for example).
The calculation hypothesis is based on two ideas. First, that some early church fathers believed the crucifixion occurred on March 25 and there is documentary evidence for this. Second, that early Christians believed prophets died on the same day they were born or conceived. This is referred to as integral age. Proponents claim there is evidence that rabbinical Judaism believed this. (Note: I have not found anything supporting this outside of their literature. There is no mention of primary sources either.) For Christians in the fourth century to also believe this means that Christian thought paralleled Jewish thought for over two hundred after Christ. If they did hold to this and believed that Christ was conceived on March 25, then nine months later would be his birth on December 25. This is a very tenuous argument and is sometimes presented as definitive by its proponents.
I should note that it is accepted knowledge that the crucifixion could not have occurred on March 25 given the information the gospel writers provided for this event. There were other dates for the birth of Christ proposed by early Christians. The spring was a popular choice including the same day as his crucifixion. Armenian Christians still consider January 6 to be the date. It is also quite possible that there were multiple reasons that December 25 was accepted.
Good resources on this topic include:
Toward the Origins of Christmas by S.K. Roll
The Origins of the Liturgical Year by Thomas J. Talley
Both of these provide a scholarly perspective with references cited as opposed to something like this.
Comments
Interest in researching and writing this was spurned by an article in World Magazine entitled Why December 25? The origin of Christmas had nothing to do with paganism.
Posted by: CJ Costello on Sunday, January 1, 2006