Which of any of the days? Actually, before Moses, who knew when the seventh day occurred?
And holy to whom beside Jehovah Elohim? I suppose Adam was created as the full-scale, adult prototype version. Day 6 is the day he met Jehovah Elohim. Who can fix it using the solar calendar, or moon cycles?
And even now, when does Sabbath begin Zulu? And where? Jerusalem? That's not the time zone in which Adam was made, was it?
Going to the Hebrew calendar, I'd kind of put my money on the idea that God fixed a Sabbath day suitable to Him then and there, and that all Jewish Sabbaths are calculated from that day both before and after it. But is the Sabbath truly a solar-based calendar day, or a moon-cycle-based calendar day. They don't jibe over a year, do they? what with Second Adar etc.
To argue over this simply does not make sense, from the sidereal time. My expectation is that only God knows (omniscient) when the seventh day after the first six actually occurred, and what we have now is solely based on human opinionated extrapolation, not fact or Scripture.
Look, we do not even have a consensus on the year Jesus died. Why do we want to get fixated on this enough to kill over it?
For Christians, as well as for Jesus and the Jewish legalistic culture, The Sabbath was made for man(kind), and not coincidentally, so was the Day of the Spirit's transition into the temporal sphere. If we go to Heaven, where there is no darkness, when is Shabbas, eh?
And if we ever get to abide in rhe Second Heaven beyond the orbit of Earth, when does day-fall come?
People need to think beyond the box.
Adam knew and handed down that information through oral tradition. Catholics are big on oral tradition so they probably already knew this. God would not give a Commandment that we have no way of completing due to lack of a calendar. He is a fair and just God. If through neglect the Sabbath was forgotten, it was re-instituted when Moses came onto the scene. We have clay calendars that go back that far, so we're in the clear. Also Jesus would have corrected it if needed. Obviously that was not the case. So we can be sure we are observing the same Sabbath that God rested on at Creation. And will do so continually throughout eternity.
The sabbath was built into the framework of our reality. It's part and parcel of an earthly existence...the cycle of night and day constitute a day...every seventh one is the sabbath.
IT was set into motion at creation...Adam and Eve knew about it. Look at this verse:
Gen 4:3 And in the process of time it came to pass that Cain brought an offering of the fruit of the ground to the LORD.
"in the process of time" is literally "at the end of days". Many scholars believe this is a reference to the 7th day sabbath...the holy sabbath created earlier by the Lord Jesus Christ. The "end of days", the last day, is the sabbath.
The sequence was likely lost to the children of Israel while in Egyptian captivity and so God (again) revealed it in Exodus 16.
NOTE that in Exodus 16 God commands his followers to KEEP the sabbath...BEFORE the old covenant was discussed or agreed upon. This makes sense since he created the sabbath BEFORE any covenants were made. So the sabbath FALLS out side of any covenants as a holy thing of God in and of itself.