If you have been living in a cave until today, welcome back, it is Christmas time. In addition to the gift giving and Santa traditions that anyone can enjoy, the holiday commemorates the birth of Jesus of Nazareth, the central figure to Christianity. Christmas, then, is a pretty important holiday for a large number of people in the world.
Christmas and the Nativity story found in the Bible books of Luke and Matthew hold a central theological purpose of illustrating the ways in which Jesus was divine as well as human. Conversations about that topic and it's enigmatic, transcendental meaning must refer back to the story of the birth of Jesus, so the holiday has held a central ritual, theological, and narrative function for Christians several centuries.
However, it has not always been celebrated on December 25, because there is little reason to believe that to be Jesus' birth date.
So, why did December 25 gain popularity? Two theories:
1) Early Christians in Rome could have adapted the holiday Saturnalia, a popular Roman mid-winter festival that celebrated the winter solstice. Christians from that time used a lot of sun/light metaphors for Jesus, so the connection could have been natural. However, this practice was not particularly popular among Christians at this particular time and place.
2) December 25 could be exactly 9 months after Mary conceived Jesus, according to some accounts.
We may never know the exact date of Jesus' birth. The Bible accounts were not that interested in the specific date so they did not give us reliable enough statistical information. Curious folks ever since have made various estimations, very few of which place Jesus' birthday in winter months at all. In fact, while Western Christians celebrate Christmas on December 25, other, equally ancient churches like Armenians and Russia celebrate in January. In fact, the first author we know about who talked about the date of Jesus' birth thinks it was in either April or May.
The first generations of Christians probably didn't celebrate Christmas at all. Divine birthdays were popular with Pagans at the time, and popular, outspoken Christians criticized the practice. Instead, they all celebrated the last supper, death and resurrection of Jesus, which they had very carefully recorded.
And remember, we know the date as December 25 because we use a modified Gregorian Calendar, which was invented in 1582 CE. That means that the dates we know and recognize are only 500 years old. And keep in mind that the trappings of a modern Christmas – tree, yule log, presents, Santa – are all directly related to Celtic and Germanic pre-Christian traditions.