Over time, the religious leaders attempted to distance themselves from the Jews (who were seen as trouble makers and the race who rejected and crucified Christ).
So Sunday became more prominent and the Sabbath became less prominent.
The first day of the week was also a day of celebration to the secular pagans in Rome – a festival in honour of the great sun-god.
Any move towards Sunday would certainly have been a wise political move.
It was the Roman Emperor Constantine, who in 321AD first passed a law to put Sunday above the Bible Sabbath.
“On the venerable Day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed.” Codex Justinianus, book 3, title 12,3; trans. in Schaff, History of the Christian Church, Vol.3, 5th edition, p380
This was confirmed in the middle of the fourth century by the church at the council of Laodicea. Here they officially claimed to transfer the blessing and holiness of the Sabbath to Sunday.
“Christians shall not Judaize and be idle on Saturday, but shall work on that day; but the Lord’s day they shall especially honor, and, as being Christians, shall,if possible, do no work on that day. If, however, they are found Judaizing, they shall be shut out from Christ.” Charles Joseph Hefele, A History of the Christian Councils, Vol.2,p316