Roman Catechism 26.23

Chapter 26 — Part III: The Decalogue — The Third Commandment

Section 23. "The Seventh Day Is The Sabbath Of The Lord Thy God" — This particular day was fixed for the worship of God, because it would not have been well to leave to a rude people the choice of a time of worship, lest, perhaps, they might have imitated the festivals of the Egyptians.