Christmas is one of the most important days of the Church year, second only to Easter itself. This year, the Fourth Sunday of Advent falls on Christmas Eve. Please keep in mind, there are two separate Mass obligations that must be fulfilled separately: one for the Fourth Sunday of Advent (Sunday, Dec. 24) and one for Christmas (Monday, Dec. 25).
Christmas is the feast of the Incarnation, the feast of God becoming flesh. The liturgical season of Christmas begins with the vigil Masses on Christmas Eve and concludes on the Feast of the Baptism of the Lord. During this season, we celebrate the birth of Christ into our world and into our hearts, and reflect on the gift of salvation that is born with him, including the fact that he was born to die for us.