Christians celebrate Christmas following a declaration by Emperor Justinian of Rome in 324 A.D., which recognized Dec. 25 as a state holiday. This declaration came after Christianity had taken over as the state religion of Rome.
Prior to 324, the festival of the Saturnalia took place from Dec. 17 through Dec. 23, at the end of which the majority of the populace broke into carnival-like behavior. As time progressed, winter solstice traditions from other cultures conquered by Rome were incorporated into the festivities. This would later include decorations involving fir trees, lights, various Yule traditions from Celtic and Germanic tribes as well as the giving of gifts.
Prior to 324, the festival was a celebration of agriculture and an homage to the Roman god Saturn, but following Justinian's declaration, it became a time for the remembrance of Christ. Since the actual date of Christ's birth was not documented or clearly known, this date would later become something of a placeholder. In 1990, a court ruling declared Christmas to be a secular holiday with no significant ties to Christianity at all. This is due to the fact that many cultures, independent of their religions, celebrate a Christmas holiday on or near the same day.