Answer:
Jewish tradition has always been Saturday was the day of Sabbath. It was changed during the first few hundred years of the early church for reasons of distancing themselves from Judaism among other reasons.
However, it is not as important what day we observe it on as long as we ALWAYS are observing the work of Christ and the freedom we have through the work of the cross.
I found this elsewhere, but it gives reason to look not to a day but to Christ…
there is one time the Sabbath is mentioned after Paul began to focus on the Gentiles, “So let no one judge you in food or in drink, or regarding a festival or a new moon or Sabbaths, which are a shadow of things to come, but the substance is of Christ” (Colossians 2:16–17). The Jewish Sabbath was abolished at the cross where Christ “wiped out the handwriting of requirements that was against us” (Colossians 2:14).
This idea is repeated more than once in the New Testament: “One person esteems one day above another; another esteems every day alike. Let each be fully convinced in his own mind. He who observes the day, observes it to the Lord; and he who does not observe the day, to the Lord he does not observe it” (Romans 14:5–6a). “But now after you have known God, or rather are known by God, how is it that you turn again to the weak and beggarly elements, to which you desire again to be in bondage? You observe days and months and seasons and years” (Galatians 4:9–10).
But some claim that a mandate by Constantine in A.D. 321 “changed” the Sabbath from Saturday to Sunday. On what day did the early church meet for worship? Scripture never mentions any Sabbath (Saturday) gatherings by believers for fellowship or worship. However, there are clear passages that mention the first day of the week. For instance, Acts 20:7 states that “on the first day of the week the disciples came together to break bread.” In 1 Corinthians 16:2 Paul urges the Corinthian believers “on the first day of the week let each one of you lay something aside, storing up as he may prosper.” Since Paul designates this offering as “service” in 2 Corinthians 9:12, this collection must have been linked with the Sunday worship service of the Christian assembly. Historically Sunday, not Saturday, was the normal meeting day for Christians in the church, and its practice dates back to the first century.
The Sabbath was given to Israel, not the church. The Sabbath is still Saturday, not Sunday, and has never been changed. But the Sabbath is part of the Old Testament Law, and Christians are free from the bondage of the Law (Galatians 4:1-26; Romans 6:14). Sabbath keeping is not required of the Christian—be it Saturday or Sunday. The first day of the week, Sunday, the Lord’s Day (Revelation 1:10) celebrates the New Creation, with Christ as our resurrected Head. We are not obligated to follow the Mosaic Sabbath—resting, but are now free to follow the risen Christ—serving. The Apostle Paul said that each individual Christian should decide whether to observe a Sabbath rest, “One man considers one day more sacred than another; another man considers every day alike. Each one should be fully convinced in his own mind” (Romans 14:5). We are to worship God every day, not just on Saturday or Sunday.