"On the venerable Day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed." (The Edict of Constantine — 321 AD)
Constantine was, like Aurelian and Diocletian before him, a worshiper of the sun. He was also the first Emperor to profess belief in Christianity. Historians debate whether or not his conversion was genuine, since he maintained his pagan superstitions throughout much of his reign.
It seems that Constantine’s personal religion was a mixture of Mithraic sun worship and Christianity. According to his Christian biographer, Eusebius, he taught all his armies to zealously honor the Lord’s day—Sunday—referring to it as “the day of light and of the sun.” This was distinctly pagan terminology.
For Christians today it may seem ironic that the first Sunday law—the famous Edict of Constantine—uses the language of sun worshipers rather than Christian expressions. The first day of the week is exalted as “the venerable day of the sun.” There is no mention of Christ or of celebrating His resurrection. That first Sunday law had no Christian flavor whatsoever.