I have a different take on Romans 14. I believe Romans 14 is saying the weaker brother is the one who only observes one day a week.
But I would like to address your post because I believe you will fall into a falsehood not supported by the Bible. You're relying on a man-made calendar to interpret the truth given to us in Scripture. The Scripture tells us we were to work six days and the rest the seventh day, keeping it holy unto the Lord. That seventh day is a Sabbath and is determined by the workweek not by the name of the days. Many countries start their calendars with Monday as the first day of the week, and Sunday as the last week. Most of the Western world starts its work week on Monday, and as I said many of the European countries also start calendar as Monday the first day of the week.
Let's review the text:
Deuteronomy 5:13 'Six days you shall labor and do all your work, 14 but the seventh day is a sabbath of the LORD your God ; in it you shall not do any work, you or your son or your daughter or your male servant or your female servant or your ox or your donkey or any of your cattle or your sojourner who stays with you, so that your male servant and your female servant may rest as well as you. Now as hard as I look within this commandment I do not see any day referred to by name. The Bible is very pacific, we are to work six days and the seventh day becomes a Sabbath. The Bible is not specific about what you should start your workweek nor what to call the days of the week. This is all a man-made trap which many have fallen into.
If you fall into this trap you must start your workweek on Sunday, and not use Sunday as a day to go to the beach, have a barbecue, or go visiting friends. You
must start your workweek on Sunday, or according to your interpretation you are committing a violation of the commandment given to us in Deuteronomy 5:13.
Let me illustrate this by using a make-believe scenario.
Let's say that somebody filed a suit with the Supreme Court of United States claiming that the names on the calendar of the days of the week were a form of religious discrimination. Because some of the days had Jewish implications and some the days had pagan implications. So let's assume that the court found on the side of the plaintiff and ordered all names be stricken of the days of the week on the calendar used in the United States. To be fair when redesigning the calendars the names of the week were just by number. In other words you had the first day of the week, the second day of the week, the third day of week, and so on to the seventh day of the week. And furthermore to make it fair and easy for the calendar printers the first day of the week would start on January 1. And to go but further the calendar would start every year with January 1 as the first day of the week.
Now obviously if you follow the calendar like this you would actually lose some sabbath days and eventually you would have 2 sabbath days back to back, December 31 and January 1.
Following the man-made calendar is a trap and
it is much better than we follow God's calendar that he made a very clear to us in Deuteronomy when he gave us the commandment, 'Six days you shall labor and do all your work, but the seventh day is a sabbath of the LORD your God'. This is a very simple commandment to follow, however some, like the Pharisees of old, want to manipulate it so they may use it for their own purposes, bringing glory and power to themselves.