Les Saturnales - Antoine-François Callet
The 25th of December hasn't always been Christmas day. The origin of the celebration of this day seems to be very ancient, but its most direct affiliation comes from, among others, the Romans, who celebrated, for a long time, the worshiping of god Saturn. Celebrations lasted for about four days, during which, noone would work, presents were exchanged, friends were visited and slaves were actually given a temporary permission to do whatever they pleased, being served upon by their masters. A king would also be crowned to play the part of Saturn. This feast was called Saturnalia and took place during the Winter Solstice.
It should be noted that the winter solstice was a very important date for agricultural economies - and the Romans were agricultural people. They would do anything to please the gods and ask them for a soft winter and the return of the sun at the beginning of spring. As Saturn was related to agriculture it is easy to understand the association between the worshipping of this god and the worshipping of the sun.
However, other worships existed as well. Such is the case of the god Apollo, god of light and the sun and very popular among the Roman army, who was celebrated during the 24th and 25th of December, which was, according to the legend, his birthday. In 273, the Aurelian Emperor established the 25th of December as the the Sun's birthday: Natalis Solis Invicti (the birth of the invincible Sun).
It's only during the 4th century that the birth of Christ begins to be celebrated among Christians (until then, they mainly celebrated Easter), but on the 6th of January, with the Epiphany. When, in 313, Constantine is converted and makes Christianity official, the Roman Church looked for a sound and broad support, looking to confuse severall pagan cults with their own. When they gave up competing with Saturnalia, they moved their holiday up a bit, incorporating the pagan feast of worshipping the Sun, transforming it into the celebration of the birth of Christ. Pope Gregory XIII did the rest: it's easier to change the calendar than to change people's desire for holidays...