In July of 321 BC, Emperor Constantine proclaimed Sunday as a day off.
The reason was that it was a day dedicated to the sun god, Sol invictus, who was widely worshiped throughout the Roman Empire at that time.
With the transition to Christianity, this day remained devoted to the Lord (Sunday). This is expressed in writing by the great ecclesiastical writer Saint Jerome a few decades after Constantine, adapting the idea of Sun light to Christian theology.
It is important for history to realize that the decisions of great people still affect our present era where Sunday is still traditionally considered a day off for those who work and one dedicated to the Lord for Christianity.
It is even more impressive that in some languages (e.g., Sun day) this decision of M. Constantine is still being reflected after many centuries.