Celebrated as the birth day of Christ, hence Christ's mass.
Christmas was generally overshadowed by Epiphany, at which the three magi announced Christ's significance. However, the date roughly coincided with the Winter Solstice, and for that reason held its own significance.
Christmas was one of the quarter days along with Lady Day on March 25th, Midsummer Day on June 24th, and Michaelmas on September 29th when rent was due. All debts had to be settled on the next quarter day.