The Catechism in Questions & Answers

320. Why do Christians hallow the Sunday as the “Sabbath”?

Christians hallow the Sunday as the“Sabbath” because Jesus resurrected  from the dead on a Sunday. Thus thefact that Christians hallow the Sunday is also a profession of the resurrection of Jesus Christ.
A reference to the significance of the Sunday as the holy day of the Christians can be found in Acts 20: 7: “Now on the first day of the week, when the disciples came together to break bread, Paul [...] spoke to them...” Here as well as in 1 Corinthians 16: 2, it is the first day of the week—the Sunday—that is set aside.