The day of worship was changed from the Sabbath (Saturday) under the Law of Moses in the time following Jesus' death and resurrection. The Ante-Nicene Fathers, Vol 1, pp 62,63
There is available today some fragmentary writings from the time of the apostle John's closing days (ca. 96A.D.) which are clearly using the phrase 'the Lord's day' to refer to the first day of the week (Matthew 28:1; Mark 16: 2; Luke 24: 1; John 20; 1. It is not surprising that such would be the case, since Jesus rose from the dead on the first day of the week. Further evidence is that Paul delayed his departure from the brethren until after he had met with them to break bread on the first day of the week (Acts 20:7-11), monetary collections were to be taken up on the first day of the week (1 Cor. 16:1-3) , and the testimony of writers of that era. Ignatius, who was a disciple of the apostle John himself, wrote:
"Be not deceived with strange doctrines, nor with old fables. For if we still live according to Jewish law, we acknowledge that we have not received grace ... those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the sabbath."
There are other writers, such as Irenaeus, the bishop of Lyons during the latter part of the second century, Clement of Alexandria, around the close of the second century, and Tertullian, at the beginning of the third century who each affirmed that the followers of Christ did not observe the sabbath. The latter writer stated:
"We have nothing to do with sabbaths or the other Jewish festivals, much less with those of the heathen." The Ante-Nicene Fathers, Vol 3, p70.
I would point out to you the great importance of a New Testament passage in which Paul argues that the Ten Commandments (of which the fourth commandment established the Sabbath observance), as such, represented "a ministration of death, written and engraven in stones ... which was to be done away" (2 Corinthians 2:7). One cannot but conclude that the sabbath was part of that which was "written and engraven in stones". Paul's comment was that the children of Israel "could not steadfastly look to the end of that which is abolished" (verse 13). This passage is a very
powerful, unambiguous, testimony against the present practice by some of the Sabbath (Saturday) observers.
This redundancy of the Law is emphasised frequently by the apostles in the New Testament. Take, for instance,
"For the priesthood being changed, there is made of necessity a change also of the law." Hebrews 7:12 "For Christ is the end of the law for righteousness to everyone who believes." Romans 10:4
Jesus had illustrated these changes.
"Ye have heard that it hath been said... But I say unto you..."
In this way, and in his teaching, Jesus affirmed and amplified nine of the Ten Commandments. The fourth commandment, relating to the sabbath, was the only one not reaffirmed by Jesus. Neither did any of the apostles. So nowhere in the New Testament is the Christian commanded to observe the Jewish sabbath.
The Seventh Day Adventists, and other 'sabbath-keepers' argue that God gave the sabbath to be kept holy at the time of creation (on the seventh day He rested) so therefore it still applies today. However God did not order man to keep that day 'holy' then, but brought it in only with the Law of Moses.
"Therefore I led them out of Egypt...I gave them my decrees and made known to them my laws, for the man who obeys will live by them. Also I gave them my Sabbaths as a sign between us, so they would know that I the Lord made them holy." Ezekiel 20:10-12.
Note the words "between US" - the sabbath was a sign between God and the nation of Israel, and this whole law for them was fulfilled and replaced by Jesus Christ.
The freedom we enjoy in Christ allows us to worship God on any day of the week. The sabbath still remains as a Saturday, but it is no longer observed as under the law. We wait for the true sabbath to come, that great time when we hope to enter "into the rest". For the true believer in Christ every day is "separated" unto God as a holy day. The first day of the week (Sunday) perhaps especially so, in the commemoration of Jesus rising from the dead to an eternal life as our mediator.