Christmas is monumental in our Catholic faith. We celebrate this feast as a pivotal moment when God became man -incarnate (the Latin "in carne" means "enfleshment") to walk with us in order for us to rise up to meet Him. Even though we have many different customs to celebrate this feast, one that is required is the celebration of mass. Whether we attend a Christmas vigil on the eve of Christmas or Christmas Day, we are required to celebrate this moment with praise and worship to our God.
It is very important that we remember why God became flesh and what He did as Jesus Christ on this earth for us. Celebrating this feast with mass allows us to remember how the Lord saves us by becoming flesh on the altar during the Eucharistic Celebration. In recognizing this we can see how Christmas and Easter are celebrated not just on designated feast days, but all year during mass.