“Did Constantine change the Sabbath from Saturday to Sunday?”……In the year 321 A.D., Constantine decreed, “On the venerable day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed”(Codex Justinianus lib. 3, tit. 12, 3; trans. in Philip Schaff, History of the Christian Church, Vol. 3, p. 380, note 1)

March 321 A.D.Constantine (<318->324) orders Sunday to be a day of rest in the cities of the Roman Empire . This is presented not as a Christian act but as a way of honoring the sun, Sol Invictus(<272)

Constantine seems to have made this change himself and not through the papacy, since the papacy had not really come in to being at that time. The papacy grew gradually out of the office of Bishop and for many years this was centered in Rome.

In any case, it should be noted that in doing this, Constantine is not changing the Sabbath; he is merely making Sunday the official day of rest for the Roman Empire. His motivation was probably not born out of hatred for the Jews (it’s hard to say for sure why Constantine or any historical figure did what they did) but out of a desire to adopt what the Christians had practiced for nearly two and a half centuries.……It is well documented that the early church adopted Sunday as their day of worship. Acts 20:7 speaks of this,

Paul's Final Visit to Troas…Acts 20:7On the first day of the week, we gathered with the local believers to share in the Lord's Supper.(Greek to break bread) Paul was preaching to them, and since he was leaving the next day, he kept talking until midnight."…1 Corinthians 16:2On the first day of each week, you should each put aside a portion of the money you have earned. Don't wait until I get there and then try to collect it all at once.

These passages indicate that Christians were probably meeting regularly on Sunday (the first day of the week). They did this most likely because Christ rose on the first day of the week. It wasn’t until hundreds of years later that the death of Christ became the focal point of Christian worship services. That is not to say they thought it unimportant; but they were primarily concerned with His victory over death realized in His resurrection.……It is important to remember that corporate worship with other believers is necessary and part of obedience, but the day that your church body chooses to worship on is not really that significant. The New Testament addresses this in a couple of different passages.

Colossians 2:14-17He canceled the record of the charges against us and took it away by nailing it to the cross.In this way, he disarmed the spiritual rulers and authorities. He shamed them publicly by his victory over them on the cross.

So don't let anyone condemn you for what you eat or drink, or for not celebrating certain holy days or new moon ceremonies or Sabbaths. For these rules are only shadows of the reality yet to come. And Christ himself is that reality.…Romans 14:5-6 In the same way, some think one day is more holy than another day, while others think every day is alike. You should each be fully convinced that whichever day you choose is acceptable. Those who worship the Lord on a special day do it to honor him. Those who eat any kind of food do so to honor the Lord, since they give thanks to God before eating. And those who refuse to eat certain foods also want to please the Lord and give thanks to God.