321 – Emperor Constantine I decreed that Sunday, the day honoring the sun god Sol Invictus(disc pictured), would be the Roman day of rest. What! You did not know?
On the venerable Day of the sun let the magistrates and people residing in cities rest, and let all workshops be closed. In the country, however, persons engaged in agriculture may freely and lawfully continue their pursuits: because it often happens that another Day is not so suitable for grain sowing or for vine planting: lest by neglecting the proper moment for such operations the bounty of heaven should be lost. (History of the Christian Church: Third Period: Nicene and post-Nicene Christianity: From Constantine the Great to Gregory the Great A.D. 311-600)