Really, the first time Christmas Day appeared on a calendar was in the Calendar of Filocalus which was part of a 4th century illuminated manuscript Chronographus Anni CCCLIV.
The pagan Roman emperor Aurelian proclaimed December 25 as "Natalis Solis Invicti," the festival of the birth of the invincible sun. The festival, shortly after the winter solstice, was celebrated with chariot races and by placing decorations on branches and small evergreen trees.
These pagan festivals remained popular for centuries after Jesus death because early Christians were unwilling to relinquish them. When the church found it impossible to abolish pagan customs, despite repeated bans, it "Christianized" a number of them.
By the 5th century Christmas Day had become an important date in the Christian year, with December 25th fixed as the "natural" date to exorcise the earlier pagan festival of the winter solstice.