Already my short answer shows that it was not a conspiracy by the early first century church or later to give up the Sabbath but that it was based on the understanding that the breaking up of the wall to the Gentiles meant also consequently not to load unto them the Sabbath.
At the same time the christian practice to celebrate the first day of the week got common, (If this was already at the beginning everywhere a weekly Lordsupper on Sunday is another question or if this derived from the domenical Easterfestival and later got celebrated weekly) ..
Already the new testament passages show as we know that on the first day of the week they met and broke the bread. Acts 13,27 shows that the apostels visited the temple on Sabbath to comment the texts that were read and to preach Jesus Christ. In some places there was a brotherly partnership and both days were held. But very early both days were distinguished. This was primarly a reaction to the jewish christians who inclined to hold the Mosaic law. .....
This all happened long before anybody intended or thought of making of this first day and celebration of the resurrection a "sunday-rest"-day, however to impose the jewish sabbath-rest to the gentiles, that was perhaps a wish of jewish christians but certainly not of the whole church.
Justin (burn about 100 A.D) explained the reason why Christians ignored the Jewish laws in a dialog with the Jew Trypho. (an antijewish apology)
Dialog with Trypho
http://www.earlychristianwritings.com/text/justinmartyr-dialoguetrypho.html
"We live not after the law, and are not circumcised in the flesh as your forefathers were, and do not observe sabbaths as you do…. An eternal and final law — namely, Christ — has been given to us…. He is the new law, and the new covenant…. The new law requires you to keep perpetual sabbath, and you, because you are idle for one day, suppose you are pious, not discerning why this has been commanded you…. If there is any perjured person or a thief among you, let him cease to be so; if any adulterer, let him repent; then he has kept the sweet and true sabbaths of God."
In Justin’s view, the Sabbath command was an admonition to morality, and Christians, by behaving morally on every day, were in perpetual obedience to the purpose of the Sabbath.
Justin repeatedly said that the patriarchs Abel, Enoch, Lot, Noah and Melchizedek, “though they kept no Sabbaths, were pleasing to God…. For if there was no need of circumcision before Abraham, or of the observance of Sabbaths, of feasts and sacrifices, before Moses; no more need is there of them now.”23 Justin argued that, since Sabbaths and sacrifices and feasts began with Moses, then they ended with Christ, who was the new covenant.24
Not only do Gentiles not have to keep the Sabbath, Justin concluded that “the just men who are descended from Jacob” do not have too, either. Trypho asked, Could a Christian keep the Sabbath if he wished to? Justin knew of some Jewish Christians who kept the Sabbath and replied, Yes, as long as he doesn’t try to force other Christians to keep the law of Moses.
The dialogue is formal in the tradition of the Platonic dialogues. It is equally aimed at Jews, Christians, and Gentiles. Justin of the martyrs here reflects his experiences as a missionary who, on his travels, probably disputed with Jewish scholars. The dialogue traces such an argument during the time of the Bar-Kochba insurrection and the subsequent antichrist riots. The fact that Justinus has actually led the dialogue in Ephesus and has also been converted to Christianity can not be historically proven.
Of course it could be that this Justin the Martyr, who was a philosopher and lived after John died., was an apostate.