The Sabbath becomes the first day of the new creation
It is obvious that a week comprises seven days: God gave us six of them on which to work and one on which to pray, take our rest and be freed from our sins… I am going to expound to you the reasons for which our tradition of keeping Sundays and abstaining from work has been transmitted to us. When the Lord entrusted the sacrament to his disciples: “He took bread, said the blessing, broke it and gave it to his disciples, saying: ‘Take, eat: this is my body, broken for you for the remission of sins.’ In the same way, he gave them the cup, saying: ‘Drink from it all of you: this is my blood, the blood of the New Covenant, shed for you and for many for the remission of sins. Do this in remembrance of me’” (Mt 26,26f.; 1Cor 11,24).
Thus the holy day of Sunday is that on which we make a memorial of the Lord. That is why it is called “the Lord’s day”. And it is, as it were, the lord of days. In fact, before the Passion of the Lord, it was not called “the Lord’s day” but “the first day”. It was on this day that the Lord established a foundation for the resurrection, that is to say he carried out the work of creation; on this day he gave the world the firstfruits of the resurrection; on this day, as we have said, he ordained the celebration of the holy mysteries. Thus this day has become a beginning for us of every grace: the beginning of the creation of the world, the beginning of the resurrection, the beginning of the week. This day, which encloses within itself three beginnings, prefigures the primacy of the Holy Trinity.
St. Teresa of Avila