The Sabbath and the Roman Catholic Church

The Catholic Church did not abandon this commandment, as some erroneously claim. Rather, observance of the third commandment to "keep holy the Sabbath" was transferred to Sunday, also known as "The Lord's Day" (Acts 20:7; 1 Corinthians 16:2), because it is through his resurrection that we become a "new creation" (2 Corinthians 5:17; Galatians 6:14). [Madrid, Patrick. Does the Bible Really Say That?: Discovering Catholic Teaching in Scripture. (St. Anthony Messenger Press: Cincinnati, OH) 2006. pp. 36 37.]
In AD 155 Saint Justin Martyr wrote a letter to the Roman emperor mentioning that the early Church celebrated the eucharistic liturgy on Sundays instead of Saturday. (ibid. p. 37)

Justin also wrote Dialogue with Trypho, which declares that we keep "perpetual Sabbath." The Lord's Day is not a replacement for the Sabbbath. It was the day Christians met and the day they celebrated the resurrection of the Lord.

Also, keeping the Sabbath is the fourth commandment, not the third.