Why is Christmas celebrated on December 25? First of all, it needs to be pointed out that in countries in which the Eastern Orthodox Church is the most prominent Christian denomination (such as Russia), Christmas--the commemoration of the birth of Jesus--is not celebrated on December 25, but on January 7, due to the fact that the Orthodox Church uses the old Julian calendar rather than the newer Gregorian calendar used by Western Christians.
One thing that can be said with certainty is that the New Testament itself does not provide any clear evidence as to what time of the year--let alone the month and/or date--Jesus was born. Some point to the reference in the Gospel of Luke to the shepherds tending their flocks of sheep and argue that this proves that the birth of Jesus occurred in the spring, during the lambing season. However, this is not conclusive. The New Testament does provide evidence for the date on which Jesus died--as we are told that His death occurred on the Friday before the Jewish Passover. Since the date of Passover is determined by the traditional Jewish lunar calendar, the date of Passover in terms of the solar calendar commonly used today will vary from year to year, but still it is at least theoretically possible to determine the actual date of the Crucifixion.
One popular explanation for why December 25 was chosen as the day on which to celebrate the birth of Jesus is that it was chosen to "Christianize" an existing pagan (non-Christian) holiday. It is pointed out that the Romans celebrated a holiday--Saturnalia--just a few days before Christmas, and that the Romans and other pagan peoples also celebrated the winter solstice around this time. Moreover, in 274 AD the Roman emperor Aurelian proclaimed December 25 as the feast of the birth of Sol Invictus ("the Unconquered Sun"). So, the theory goes, the early Christians appropriated the date of December 25 to commemorate Jesus' birth in order to make Christianity more appealing to non-Christians. This was an argument made by some 18th and 19th century Bible scholars inspired by the new field of comparative religion, according to Andrew McGowan in an article at Biblical Archeology Review ("How December 25 Became Christmas")
As Valerie Strauss of the Washington Post points out (see here), this theory--that the date of Christmas was of pagan origin--was elaborated on in James George Frazier's The Golden Bough, first published in 1890. In his book, Frazier, an anthropologist, connected the celebration of the birth of Jesus with pagan celebrations of a virgin goddess giving birth to the sun at the winter solstice. According to Frasier:
Thus it appears that the Christian Church chose to celebrate the birth of its Founder on the twenty-fifth of December in order to transfer the devotion of the heathen to the Sun to him who was called the Sun of Righteousness. (as quoted in Strauss)
However, this popular notion that the date of Christmas was pagan in origin is problematic. As Andrew McGowan notes in the above-mentioned article, while ancient Christian writers did see a connection between the winter solstice and the birth of Jesus, they "clearly [didn't] think the date was chosen by church. Rather, they [saw] the coincidence as a providential sign, as natural proof that God had selected Jesus over the false pagan gods." Moreover, the first mention of December 25 as the date of the birth of Jesus comes in a document dating from the mid-4th century. It was not until the 12th century that we see a Biblical commentator asserting that the date of Christmas was chosen to supplant a pagan holiday. Furthermore, while it is true that starting in the mid-fourth century the Church began to "Christianize" pagan festivals, "the first mention of a date for Christmas (c. 200) and the earliest celebrations that we know about (c. 250-300) come in a period when Christians were not borrowing heavily from pagan traditions of such an obvious character" (McGowan).
An alternative explanation for why December 25 was chosen as the date on which to celebrate the birth of Jesus is rooted in an ancient Jewish belief that "great things might be expected, again and again, at the same time of the year" (McGowan). It appears that many early Christians came to believe that Jesus died on the same day that He had been conceived, as was thought to have been the case with the Jewish prophets. Based on certain calculations, it was believed that the date of the Crucifixion would have been March 25, and so Jesus' conception would also have occurred on March 25. Counting off exactly nine months (for Mary's pregnancy) from March 25 would bring us to December 25. Interestingly, though, while Christians in the western Roman Empire accepted December 25 as the date for Christmas, some in the eastern Roman Empire chose to celebrate Christmas on January 6. Today, January 6 is celebrated by many Christians as Epiphany, commemorating the arrival of the Magi to present gifts to the young Jesus. The twelve days between December 25 and January 6 are the traditional Twelve Days of Christmas.
In short, we cannot be certain as to why December 25 was chosen for the celebration of Jesus' birth. So, the next time someone confidently tells you that it is a fact that Christmas is celebrated on December 25 because the early Christians copied the date of a pagan holiday, you can respond: "Actually, it's complicated!"
Image of the 2014 National Christmas Tree in Washington, DC, from Wikimedia Commons