Exodus 20:8-11 commands the seventh day, the only day of rest in antiquity with a stated theological reason. Constantine's law, preserved in the Codex Justinianus 3.12.2, orders judges, city people, and craftsmen to rest on the venerable day of the Sun, while explicitly letting farm work continue. The second layer is industrial: Ford Motor Company instituted the five-day week on September 25, 1926, and the Fair Labor Standards Act of 1938 capped the American week at forty hours, effective 1940.
The 321 edict does not say the Lord's Day. It says dies Solis, the day of the Sun, and one camp has read that for centuries as a sun-worshipper's fingerprint: the emperor honoring Sol Invictus, whose coins he was still minting. The other camp reads it as a civil law written for a pagan empire, giving cover to Christians who were already gathering on that day. The text supports both readings, which is why the fight has lasted seventeen centuries.
The hunt for the Sabbath's ancestor keeps landing on the Babylonian shapattu, the full-moon day of the fifteenth, a day of omens and propitiation. The resemblance is phonetic and that is nearly all: shapattu was monthly, tied to the moon, and no day of rest, while the Sabbath is weekly, lunar-free, and rest-defined. The scholars who insist on the loan and the scholars who reject it have been deadlocked for a century, because the tablet record shows the word but not the institution.
Your week holds two holy days: one commanded by God, one legislated by an emperor, one bought by unions and a carmaker. When the third day off arrives, whose name will be on it?