Roman Catholics observe Sunday as the day of worship because they changed the day of worship from Saturday to Sunday. Their explanation follows below:

The quotation listed below comes from: The Convert's Catechism of Catholic Doctrine by Rev. Peter Geirmann, C.SS.R., Imprimatur Joseph E. Ritter S.T.D. Archbishop of St. Lois, B. Herder Book Co., St. Lois, MO.

Q. What is the Third Commandment?*

A. The Third Commandment is: Remember that thou keep holy the Sabbath day.

Q. Which is the Sabbath day?

A. Saturday is the Sabbath day.

Q. Why do we observe Sunday instead of Saturday?

A. We observe Sunday because the Catholic Church transferred the solemnity from Saturday to Sunday.

Q. Why did the Catholic Church substitute Sunday for Saturday?

A. The Church substituted Sunday for Saturday, because Christ rose from the dead on a Sunday, and the Holy Ghost descended upon the Apostles on a Sunday.

Q. By what authority did the Church substitute Sunday for Saturday?

A. The Church substituted Sunday for Saturday by the plenitude of that divine power which Jesus Christ bestowed upon her.

Q. What does the Third Commandment command?

A. The Third Commandment commands us to sanctify Sunday as the Lord's Day.

Q. What does the Third Commandment forbid?

A. The Third Commandment forbids (1) The omission of prayer and divine worship; (2) All unnecessary servile work; (3) Whatever hinders the keeping of the Lord's Day holy.

Q. Is the desecration of the Lord's Day a grievous matter?

A. The desecration of the Lord's Day is a grievous matter in itself, though it admits of light matter.

Note: In the King James Version of the Bible, the Sabbath Commandment is the 4th Commandment. See Exodus 20:8-11. Since the Commandments are not numbered, the Roman Catholics state: "The division and numbering of the Commandments have varied in the course of history. The present catechism follows the division of the Commandments established by St. Augustine, which has become traditional in the Catholic Church. It is also that of the Lutheran confessions. The Greek Fathers worked out a slightly different division, which is found in the Orthodox Churches and Reformed communities."Catechism of the Catholic Church" 1994. p.557.