On this day (March 7th) the Roman Emperor Constantine enacted his law which obliged all the citizens of the Roman Empire to keep the “venerable day of the sun”. The practice of some third century Christians to keep Sunday, the first day of the week, gave the idea to the Roman emperor to use that custom to seal unity within the empire. He retained the old pagan term in his law – ‘Dies Solis” (in Latin “the day of the sun”, or “Sunday”) – which came into force on March 7, 321 A.D. Constantine’s Sunday law stipulated the following:
“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”.
Constantine’s ecclesiastical Council of Nicaea in 325 A.D. abolished both the apostolic day of rest Sabbath and Passover. Constantine describes his motives behind such a decision:
“ it appeared an unworthy thing that in the celebration of this most holy feast (Easter) we should follow the practice of the Jews, who have impiously defiled their hands with enormous sin, and are, therefore, deservedly afflicted with blindness of soul. … Let us then have nothing in common with the detestable Jewish crowd; for we have received from our Savior a different way”.
Church historian Moshaim writes that fourth century superstitions gradually supplanted true piety. He attributes the shameful transformation of Christianity into paganism to a crazy drive to blend pagan rites with Christianity. Thus, beginning from Constantine, a new religion emerged of which “neither Jesus nor Paul were the founding fathers”. “As a completely new religion in the Roman Empire, Christianity came as the result of later interpretations of the original traditions about Christ’s life, teachings, death and resurrection”. 
Constantine built “St. Peter’s” basilica in Vatican on an ancient pagan cemetery. He died before completing the Church of the Twelve in Constantinople, where he planned his tomb surrounded by the tombs of the apostle. “The first Christian Emperor” dreamed to rest forever amidst the remains of the Twelve not like one of them, but as a symbol of their leader. 
1 Milan Vukomanovic, Early Christianity – from Jesus to Christ, Novi Sad: Svetovi, 1996, 19.
2 William Steuart McBirnie, The Search for the Twelve apostles, Carol Stream, Illinois: Tyndale House Publishers Inc., 2004, 18-19.