You may have heard the story of how Constantine changed the Sabbath to Sunday. According to the story, the early churches kept the Sabbath until Constantine, who was the high priest of paganism and who honored the sun god, changed the Sabbath, the 7th day, to the day of the sun, the 1st day.
The story isn't true. (How do we know?)
If you came to this page from my Sabbath page, then you know that the churches prior to Constantine didn't keep the Jewish Sabbath. They did not refrain from work on the seventh day of the week (or on the 1st day, either), so there was no Sabbath-keeping for Constantine to put an end to.
There was an issue that Constantine and the Council of Nicea did have to address that concerned Sunday. That issue had been debated for at least two centuries prior to Constantine's day …
The early churches observed passover each year, which they called pascha in Greek, a word meaning suffering and referring to Christ's suffering before and upon the cross.
There was a question as to whether it was best to observe Passover on Nisan 14, the day the Jews celebrated it, no matter which day of the week it fell on, or whether to observe it on the Sunday nearest Nisan 14. The early church made a special day of Sunday, but not because it was a day consecrated to the sun god, as is often suggested by Sabbath-keepers. Instead, they consecrated Sunday as the Lord's Day, the day on which Jesus rose from the dead.
The Lord's day was not a day of rest but a day of rejoicing. It was a tradition with the early churches not to kneel on Sunday because it was the day on which Jesus rose from the dead. Thus, it was to be a day of celebration, and they did not kneel or fast (De Corona 3).
It was very difficult to settle the Passover question.
The early churches had two ways of settling controversies. One was to resort to the Scriptures. The other was to consult the tradition the apostles had given to the churches.
Paul assigns great importance to such tradition, telling both the Corinthians (1 Cor. 11:2) and the Thessalonians (2 Thess. 2:15) to hold fast to his traditions. He specifically told the Thessalonians that they were to do so whether the traditions were written or verbal.
In this case, however, the Scriptures had nothing to say, and the traditions handed down to the various churches differed:
When the Council of Nicea met in A.D. 325, the controversy had never been settled. The west still held to one practice, and the east another.
The Council of Nicea—a council attended and somewhat led by Constantine—did make a decision for Sunday, but not to change the Sabbath to Sunday. Instead, they ruled only on the question of the celebration of Passover. The church had been meeting on Sunday for centuries.
It should be obvious that the Lord's Day is the first day of the week, but people have argued and written books to the effect that the Lord's day is the Sabbath. These are the same people who have produced the myths about Constantine changing the Sabbath to Sunday and the fabricated history about the early Church keeping the Sabbath.
As you can see, Ignatius contrasts the Lord's day with the Sabbath, and he tells us that it is the day "on which also our life has sprung up again by him." He is referring to the first day of the week, which we now call Sunday.
Justin doesn't bother referring to the Lord's day or the first day of the week. A Roman living in Rome and writing to a Roman emperor, he is content to refer to the day in Roman terminology: the day of the sun, or Sunday.
As you can see, Christians had been meeting on Sunday since the first century.
So here is what the Council of Nicea did decree. This is from the synodal letter sent out after the council:
As you can see, The only decision the Council of Nicea made about Sunday was that Passover would be celebrated on the Sunday nearest Nisan 14, rather than on Nisan 14 itself. The idea that Constantine and the Council of Nicea changed the Sabbath to Sunday from Saturday is simply a myth.
It is important to point out that Constantine did make an edict, in 324, the year before the Council of Nicea, mandating worship of the Supreme God on Sunday (Gonzalez, Justo, The Story of Christianity, p. 123). This could be seen as honoring Christians, for whom Sunday was the Lord's day, but it could also be seen as honoring the sun god as well.
Either way, the idea that Constantine or the Council of Nicea changed the Sabbath to Sunday from Saturday is simply false. The Christian Sabbath was never Saturday or any other day of the week, so there was nothing for Constantine to change.