HE IS RISEN!
"By a tradition handed down from the apostles which took its origin from the very day of Christ's Resurrection, the Church celebrates the Paschal mystery every seventh day, which day is appropriately called the Lord's Day or Sunday."36 The day of Christ's Resurrection is both the first day of the week, the memorial of the first day of creation, and the "eighth day," on which Christ after his "rest" on the great sabbath inaugurates the "day that the Lord has made," the "day that knows no evening."37 The Lord's Supper is its center, for there the whole community of the faithful encounters the risen Lord who invites them to his banquet:38
The Lord's day, the day of Resurrection, the day of Christians, is our day. It is called the Lord's day because on it the Lord rose victorious to the Father. If pagans call it the "day of the sun," we willingly agree, for today the light of the world is raised, today is revealed the sun of justice with healing in his rays.39
From the Catechism of the Catholic Church