The 24/7 retail world has gutted any real observance of the 3rd Commandment (Luther's numbering, 4th for everyone else) so it is no surprise that trying to maintain any other days as holy is a real upstream swim. The sad truth is that many children are growing up believing that Church "happens" only on Sundays, except perhaps for Christmas Eve.
The 3rd Commandment, "Thou shalt keep holy the Sabbath Day," is the Catholic Church's traditional enumeration. It was the Protestants who protested (thus their title) and divided the first Commandment into two. Even their resulting first and second were regarding the same principle, the worship of the One True God (and not false gods, including graven images).
When they were done, the erstwhile 3rd became the new 4th. The old fourth (and the one traditional Catholics still use) is "Thou shalt honor thy father and thy mother."
So when you see "sins against the 6th and 9th Commandments" in the writings of the Fathers, saints and Doctors of the Church, you are reading about the sins of the flesh, transgressions against "Thou shalt not commit adultery," and "Thou shalt not covet thy neighbor's wife." But if you're Protestant, you would not understand this, thinking that there is not continuity with the writing and the two Commandments: "Thou shalt not kill" and "Thou shalt not bear false witness against thy neighbor." As a Protestant, one would likely think these, the 5th and 8th are the 6th and 9th, since the 1st had been cleverly divided and the 9th and 10th combined, bumping down the second through the 9th by one.
They combined the 9th and 10th into one, in order to maintain the number at 10, instead of having to call it the 11 Commandments. Too much change can be a bad thing, apparently. You've got to draw the line SOMEWHERE! It has always made me wonder how the "feminists" don't make that move a big deal because it effectively lumps wives in with chickens, goats, boats and motorcycles, as "neighbor's goods." What feminist wants to be chattel? That's more like a Mohammedan theme than a Christian one!
You said, "Luther's numbering, 4th for everyone else." Everyone else does not include traditional Catholics, so perhaps you are figuring they are "nobodies." I don't really know, but perhaps you are misled by the fact that all Novus Ordo parishes that I have encountered in the past 10 years have fallen into the new enumeration of the Protestants regarding not only the 10 Commandments but also the Psalms. The way the Psalms have been jostled around in the KJV and the NAB, etc., is quite disturbing. But that would take two more pages to cover...