It's a matter of historical record:
Constantine decreed (March 7, 321) dies Solis—day of the sun, "Sunday"—as the Roman day of rest just prior to the Council of Nicaea:
“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 suitable for grain-sowing or vine planting; lest by neglecting the proper moment for such operations the bounty of heaven should be lost.”
Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ. (Canon 29 [A.D. 360]). (The Church Council of Laodicea circa 364 CE)