After the death of Jesus, it was the First day. See Acts 20.
Acts 20:7 And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.
The Jewish day started at evening. Thus, in order for Paul to have been preaching on the first day of the week until midnight, it would have had to be what we consider Saturday night. Thus Paul preached on what we consider Saturday night until midnight, planning to depart Sunday.
Even if though, Paul had preached on Sunday morning, this would be no proof that the fourth commandment had been done away with. If this one verse in the book of Acts means the first day had replaced the seventh day as the Sabbath in the new covenant, then these next four verses certainly in the book of Acts certainly establish the seventh day Sabbath in the same.
Acts 13:14 But when they departed from Perga, they came to Antioch in Pisidia, and went into the synagogue on the sabbath day, and sat down.
15 And after the reading of the law and the prophets the rulers of the synagogue sent unto them, saying, Ye men and brethren, if ye have any word of exhortation for the people, say on. 16 Then Paul stood up, and beckoning with his hand said, Men of Israel, and ye that fear God, give audience.
Here Paul preached on the Sabbath in the synagogue. I guess according to your mind set, this means we should be going to the synagogue on Sabbath still.
Acts 13:42 And when the Jews were gone out of the synagogue, the Gentiles besought that these words might be preached to them the next sabbath.
43 Now when the congregation was broken up, many of the Jews and religious proselytes followed Paul and Barnabas: who, speaking to them, persuaded them to continue in the grace of God. 44 And the next sabbath day came almost the whole city together to hear the word of God.
Of course, the above verses according to your mind set, mean that the gentiles which accepted Christ, still observed the seventh day Sabbath, and so should we. I have no argument with that.
Acts 17:1 Now when they had passed through Amphipolis and Apollonia, they came to Thessalonica, where was a synagogue of the Jews: 2 And Paul, as his manner was, went in unto them, and three sabbath days reasoned with them out of the scriptures,
3 Opening and alleging, that Christ must needs have suffered, and risen again from the dead; and that this Jesus, whom I preach unto you, is Christ. 4 And some of them believed, and consorted with Paul and Silas; and of the devout Greeks a great multitude, and of the chief women not a few.
Here we have Paul preaching not just one Sabbath, but three in a row. Not only Jews accepted the message, but a great multitude of Greeks. Of course this is far more, and conclusive proof that the Sabbath was still being observed during the time of the Apostles, than the one obscure proof text which you have offered.
Acts 18:3 And because he was of the same craft, he abode with them, and wrought: for by their occupation they were tentmakers. 4 And he reasoned in the synagogue every sabbath, and persuaded the Jews and the Greeks.
More of the same. Paul reasoned with the Jews and Greeks every Sabbath in the synagogue while in Corinth. According to verse eleven of the same chapter Paul stayed there a year and six months teaching the word of God among them. Surely, if as you suggest, the mere mention of one preaching on a certain day is evidence that that day was the Sabbath of the new covenant, then by your own reasoning, the seventh day is still the Sabbath of the new covenant. Of course I feel sure that you will now modify your reasoning. Or, perhaps I have misunderstood your reasoning. If so, my apologies, and please do explain further.