THE Reformer did not teach that the Christian Sabbath must be on Sunday

¹Now concerning the collection for the saints, as I have given orders to the churches of Galatia, so you must do also: ² On the first day of the week let each one of you lay something aside, storing up as he may prosper, that there be no collections when I come. (1 Cor 16:1-2., NKJV)

The literal translation of the start of verse 2 is along the lines of “On one of the Sabbaths.” By tradition, it gets translated as “on the first day of the week” and is then taken (incorrectly, in my opinion) to mean that Paul is prescribing that Sunday is to be the Christian Sabbath. John Calvin disagreed. He argued that what Paul meant was the literal: That on one of the Sabbaths (or more) before Paul arrives they should collect an offering for Jerusalem. A day of the week is not in mind. Given that Paul was coming at some indeterminate time in the future and not the following week, the literal interpretation is a better fit.

Calvin writes:

The clause rendered on one of the Sabbaths, (κατὰ μίαν σαββάτων,) Chrysostom explains to mean — the first Sabbath. In this I do not agree with him; for Paul means rather that they should contribute, one on one Sabbath and another on another; or even each of them every Sabbath, if they chose. For he has an eye, first of all, to convenience, and farther, that the sacred assembly, in which the communion of saints is celebrated, might be an additional spur to them. Nor am I more inclined to admit the view taken by Chrysostom — that the term Sabbath is employed here to mean the Lord’s day, (Revelation 1:10,) for the probability is, that the Apostles, at the beginning, retained the day that was already in use, but that afterwards, constrained by the superstition of the Jews, they set aside that day, and substituted another. Now the Lord’s day was made choice of, chiefly because our Lord’s resurrection put an end to the shadows of the law. Hence the day itself puts us in mind of our Christian liberty. We may, however, very readily infer from this passage, that believers have always had a certain day of rest from labor — not as if the worship of God consisted in idleness, but because it is of importance for the common harmony, that a certain day should be appointed for holding sacred assemblies, as they cannot be held every day. For as to Paul’s forbidding elsewhere (Galatians 4:10) that any distinction should be made between one day and another, that must be understood to be with a view to religion, and not with a view to polity or external order. (Calvin's Commentary on 1 Cor., emphasis added)

Regarding the Lord’s Day, Calvin is arguing that the early church held it on Saturday, but to avoid confusion with the Jewish Holy Day they moved it, as one might naturally expect if you are going to move it, to the next day, Sunday, or the first day. Not because scripture mandated that the day of corporate worship to coincide with the day of the week of our Lord's resurrection (although that's certainly a wonderful alignment) but because it was convenient.  In that view, this passage:

Now on the first day of the week, when the disciples came together to break bread, Paul, ready to depart the next day, spoke to them and continued his message until midnight. (Acts 20:7, NKJV)

is describing, not prescribing. Which is how it reads.

R. C. Sproul was, as far as I can tell, in agreement with Calvin and summarizes Calvin’s position this way:

John Calvin argued that it would be legitimate to have the Sabbath day on any day if all of the churches would agree, because the principle in view was the regular assembling of the saints for corporate worship and for the observation of rest.

Modern uber-reformed confessionalists argue that worship must be on Sunday, not as a matter of convenience or tradition (which are perfectly fine reasons to hold worship services on Sunday, not to mention most people have the day off) but for incorrect legalism-- that is they falsely teach that scripture calls for Sunday worship. What choice do they have? Many take an all or nothing approach the a giant uninspired confession that calls for prescribed Sunday worship, with scriptural proof texts that, is often the case, fall short of living up to their billing. 

Many of the uber-reformed agree with Calvin except when they don't, such as his position on the necessity of Sunday Worship, the perpetual virginity of Mary (and consequently that Jesus had no blood brothers--Calvin affirmed this) and, if they are Baptists, on paedobaptism. When they lament "what has happened to the Reformation?" what they really mean is "What has happened the part of the Reformation we agree with?"