The day of the Sun. Sun worship originated with the death of Nimrod. His wife exclaimed that he died, went into the sun and impregnated her with his rays. Sun worship was later adopted by Babylon and, eventually Rome, where Constantine mingled sun worship with Christianity.
On 7 March 321, Constantine I, Rome’s first Christian Emperor (see Constantine I and Christianity), decreed that Sunday would be observed as the Roman day of rest:[10] He is quoted as saying:
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 vine-planting; lest by neglecting the proper moment for such operations the bounty of heaven should be lost.
Constantine
Once this decree was given, a slow methodical approach was enacted which resulted in most of the Christian world adopting this new day of rest with no Biblical foundation.