That first quote you have there was not my statement. It is an incomplete cut of the discussion I was having with Grandpa. That sounds weird to say, as if my grandpa and I...
692 Lex Constantini a. 321 (Cod. Just. l. iii., Tit. 12, 3): Imperator Constantinus Aug. Helpidio: "Omnes judices, urbanaeque plebes et cunctarum artium officia venerabili die Solis quiescant. Ruri tamen positi agrorum culturae libere licenterque inserviant, quoniam frequenter evenit, ut non aptius alio die frumenta sulcis aut vineae scrobibus mandentur, ne occasione momenti pereat commoditas coelesti provisione concessa. Dat. Non. Mart. Crispo ii. et Constantino ii. Coss." In English:
"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. (Given the 7th day of March, Crispus and Constantine being consuls each of them for the second time.)" The prohibition of military exercises is mentioned by Eusebius, Vita Const. IV. 19, 20, and seems to refer to a somewhat later period. In this point Constantine was in advance of modern Christian princes, who prefer Sunday for parades.
http://www.bible.ca/history/philip-schaff/3_ch07.htm