Answer: The seventh-day Sabbath has never changed. Over the years, kings and religious leaders have changed the day of worship to Sunday. If you are a history buff you can look it up.
Constantine is generally credited with applying Sunday worship to Christianity in order to get the majority of the non-believers to convert to the Sunday-keeping church. After defeating his enemies and becoming Emperor of Rome, Constantine presided in self-important splendor over the First Council of Nicaea in 325 A.D. As a shrewd politician, his scheme was to unite Christianity and paganism in an effort to strengthen his disintegrating empire. Constantine knew that non-religious people throughout the empire worshiped the sun on “the first day of the week,” and he discovered that many professing Christians—especially in Rome and Alexandria—also kept Sunday because they assumed that Christ rose from the dead on that day. So, Constantine developed a plan to unite both groups on the common platform of Sunday-keeping. On March 7, 321 A.D., he passed his famous national Sunday law:
First Sunday Law enacted by Emperor Constantine, March 321 A.D.
“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. (Given the 7th day of March, Crispus and Constantine being consuls each of them for the second time [A.D. 321].)” Source: Codex Justinianus, lib. 3, tit. 12, 3; trans. in Philip Schaff, History of the Christian Church, Vol. 3 (5th ed.; New York: Scribner, 1902), p.380, note 1.
The apostles, disciples and the entire early church founded by Jesus Christ kept the Sabbath on Saturday. Christ did not change the day of worship or modify the Fourth Commandment. We read in Hebrews 13:7-8, "Remember those who rule over you, who have spoken the word of God to you, whose faith follow, considering the outcome of their conduct. 8 Jesus Christ is the same yesterday, today, and forever."
Christ kept the Sabbath and taught on the Sabbath as it was His custom. Luke 4:16, "So He came to Nazareth, where He had been brought up. And as His custom was, He went into the synagogue on the Sabbath day, and stood up to read."
Even after Christ’s death the Apostle Paul continued to observe and preach on the Sabbath. If a change to Sunday-worship was supposed to take place… the Apostle Paul never got the message! Acts 17:2, "Then Paul, as his custom was, went in to them, and for three Sabbaths reasoned with them from the Scriptures. (Acts 18:4)