The point was not the first day, but the closing of their worship day, the Sabbath, that was the center of their activities. Every mention of the first day in the NT is of this nature. The first day never had any significance to them of its own.
No, the Holy Spirit focus on the beginning of the first day, and it is during that time that the preaching service takes place, and on that day the Lord also met with disciples after the resurrection. But that you do either day to honor the Lord is the main thing, unless you think those do not keep the 7th day are all lost.