What about Passover? Where is it ever commanded to be done away with and substituted with your Friday afternoon/Sunday morning affair? Here are your "early fathers" discussing this problem.
I think it is important to note that Polycarp, Bishop of Smyrna and a disciple of the Apostle John, insisted upon the proper observation of Passover on the 14th. So did all of the other Apostles and early Christians. Where does your organization get the right to change the timing of a Holy Sabbath of the Lord?
The celebration of Easter varied. Who denies this? But the Council of Nicaea, held in 325, regularized the feast day to be universally observed along the pattern of the Church at Rome, the matter was settled, and the Church moved on. Nothing doctrinal was at stake, it was merely a matter of custom needing to be regularized.
As for your question on the Sabbath day, the answer is simple: the Church decided that the people would meet in common on the first day of the week - the day of the Lord, commemorating His resurrection. Exactly *when* was this decided? Who knows? It was already an established sustom in St. Paul's time. St. Paul is involved in two things that are clues.
First, in Acts 20:7 St. Paul is in Troas, where the Christian community gathered together to break the Bread on the first day of the week (Sunday).
Second, you will note that in 1Corinthians 16:02, St. Paul solicits an *ongoing* collection to be taken up for the Christian community in Jerusalem. This is to be undertaken for successive weeks before his arrival in Corinth, and it is to be taken up "on the first day of every week." Why? Because THAT'S the day the Christians are assembling *anyway* for the breaking of the Bread.
A third clue comes from Revelation 1:10, when St. John talks about being in the Spirit on "the Lord's day." This was Sunday. Among other VERY early witnesses to this fact is St. Ignatius of Antioch, who, on his way to martyrdom in 107 AD, writes to the church in Magnesia, and speaks about, "no longer observing the Sabbath (Saturday), but living in the observance of the Lord's day, on which also Our Life rose again" (obviously a reference to Sunday). (Letter to the Magnesians, 9) There are other references to the Sunday vs. Saturday issue among the Fathers, and they all speak of it as a long since settled matter. I don't have them handy now, but I know there is something in the Epistle of Barnabas, and in St. Justin Martyr's writings. Both of these are second Century.
I would grant that the three Biblical citations are nowhere near "proof-text" grade stuff. But that underscores two points. First, the Church VERY early on had evidently decided on Sunday to have their agape meal. The decision came so early that there is no direct mention of it in the New Testament. It was already established practice by the time St. Paul and the other NT writers wrote their letters to the various churches; the people already knew about it through "oral tradition," and it was not necessary to belabor what they already knew. Second, while there is at least shadowy evidence for an early Sunday worship in the NT based on the references cited, there is NOTHING else subsequent to this in any Christian writing, council or whatever, that *determines* Sunday to be the day of worship. That in itself points to such an early beginning of the Sunday worship day that it must go back to the Beginning. As this is likely, it also raises a third point: here is some evidence that not *everything* known about the first-generation Christian Church can be found in the New Testament, as some seem to believe.
Bottom line: the Church had the authority to bind and to loose "whatsoever" it willed, as granted by Jesus and as led by the Spirit. At some reeeeeeeeaaaaalllllyyy early date, the decision was made to observe the Day of the Resurrection as the new Sabbath day. By the time of the destruction of the Temple, all vestiges of Sabbath worship among Jewish Christian converts had disappeared, and even then, they were really acting as Jews, not Christians, because there was still inner confusion (soon dispelled) as to whether Christianity was within Judaism or a separate religion budding off from it. Before the end of the first Century, there is not a trace of this practice remaining.