Catholics do not worship on the Sabbath which is the last day of the week according to Jewish teaching, when God rested from work after creation (Gen. 2:2-3). We worship on Sunday, the Lord’s Day, the eight day (or first day of the week) when God created Light (Gen 1:3); the day of the Lord’s resurrection; the day the Holy Spirit came on the Apostles.
"The Church celebrates the day of Christ’s Resurrection on the ‘eighth day,’ Sunday, which is rightly called the Lord’s Day" (CCC 2191). "The Sabbath, which represented the completion of the first creation, has been replaced by Sunday, which recalls the new creation inaugurated by the Resurrection of Christ" (CCC 2190).
From the NT Sunday not Saturday was kept as the Day to be observed:
- On the first day of the week when we gathered to break bread, Paul spoke to them because he was going to leave on the next day, and he kept on speaking until midnight (Acts 20:7).
- On the first day of the week each of you should set aside and save whatever one can afford, so that collections will not be going on when I come (1 Cor. 16:2).
- Let no one, then, pass judgment on you in matters of food and drink or with regard to a festival or new moon or Sabbath (Col. 2:16).
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. 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." (CCC 1166)