Emperor Constantine I decreed that courts and city workers should cease labor on 'the venerable day of the Sun,' with exceptions for agricultural work. The measure blended the emerging Christian observance of the Sabbath with the empire's existing sun-worship traditions, an early sign of Rome's gradual transformation into a Christian state and a decision whose seven-day rhythm still governs modern calendars.
Constantine's edict, recorded in the Codex Justinianus, did not explicitly invoke Christianity—it spoke of the 'venerable day of the Sun,' a formulation that satisfied both his newly favored Christian subjects and the empire's many worshippers of Sol Invictus. This deliberate ambiguity was characteristic of Constantine's careful political balancing act as he steered a vast, religiously diverse empire toward a faith he himself had only recently embraced after his vision before the Battle of the Milvian Bridge.
The practical effect was enormous and durable. Judges, city dwellers, and tradesmen were ordered to rest, though farmers were explicitly permitted to keep working since crops don't wait for legislation. This is often cited as the origin of the Western weekend and the seven-day week structure that spread with Christianity and Roman administrative practice across Europe and eventually the globe, making March 7, 321 a quiet but foundational date in the history of everyday life.