Or, as it should more properly be said, Io Saturnalia!
Because Christmas is a pagan Roman festival in honour of the god Saturn. The Io is pronounced Yo, making ancient Romans sound somewhat like rappers. Saturnalia was appropriated by early Christians for the birth of Jesus, because there wasn't the slightest hope of getting people to give up their beloved Saturnalia.
Saturnalia was the Time of Misrule. All normal order disappeared. Masters were expected to serve their own slaves. Saturnalia began on what we'd call 17th December and carried on for a week or more of non-stop partying. Several Roman emperors tried to limit the time of Saturnalia, but everyone ignored them.
The official last day of Saturnalia was our 23rd December, called Sigillaria, when people gave each other gifts.
Does this sound familiar? Christmas/Saturnalia is such an ancient festival that no one has the faintest clue when it first began. When you gather round on Christmas Day with your friends and family and exchange gifts and happiness, you'll be carrying on a human tradition that's been going for thousands of years.
Keep up the good work.