Catholic tradition refers to Sunday as the "Lord’s Day." The only reference of the Lord’s Day in scripture is in Revelation 1:10, “I was in the Spirit on the Lord's day….” Let us see, according to Scripture, which day is defined as the Lord’s Day [Master's Day]: “For the Son of man is Lord [Master] even of the sabbath day.” Matthew 12:8. Seeing that the Master’s Day is the Sabbath day, let us again look to scripture to see which day is the Sabbath day: “But the seventh day is the sabbath of Yahuwah thy Eloah.” Exodus 20:10.
In His word, the all-knowing Yahuwah clearly differentiated the Sabbath day from the first day of the week, and never uses both terms simultaneously to refer to the same day. Acts 13: 42, 44, 16:3, 17:2, 18:4 refer to the Sabbath day; Acts 20:7, 1 Corinthians 16:2 refer to the first day of the week.