THERE IS NO "Sabbath day" for the Christian.
The "Sabbath rest was a "type" of the rest from our works that we have in Jesus. Since he rose from the grave, JESUS is the "Christian Sabbath" and we rest CONTINUALLY in Him from our own filthy "works of righteousness".
The weekly "Sabbath day" (there were other "Sabbath days") starts at sundown Friday, and continues till sundown saturday. That was NEVER changed. SUnday was NEVER the "Sabbath" in any way whatsoever (although church traditions often did attach all sorts of silly superstitious restrictions to it).
Messianic groups and other "Sabbatarian" denominations make a big deal about it (there are even "Seventh day Baptists" that meet on Saturday - go figure).
But the answer is really quite simple - then the "gentile dogs" were included into the Church (house of Cornelius) the question became - do we make THESE folks (shudder) ascribe to Jewish customs, or not????
The answer was: Acts 15:
23 And they wrote letters by them after this manner; The apostles and elders and brethren send greeting unto the brethren which are of the Gentiles in Antioch and Syria and Cilicia:
24 Forasmuch as we have heard, that certain which went out from us have troubled you with words, subverting your souls, saying, Ye must be circumcised, and keep the law: to whom we gave no such commandment:
25 It seemed good unto us, being assembled with one accord, to send chosen men unto you with our beloved Barnabas and Paul,
26 Men that have hazarded their lives for the name of our Lord Jesus Christ.
27 We have sent therefore Judas and Silas, who shall also tell you the same things by mouth.
28 For it seemed good to the Holy Ghost, and to us, to lay upon you no greater burden than these necessary things;
29 That ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication: from which if ye keep yourselves, ye shall do well. Fare ye well.
The Gentiles never did keep the "Sabbath", and this specifically absolves them from bothering with it.
And that just about covers it.