Changing the day of Shabbos?

Question

There are many arguments for calendar reform, one of which argues that we should set up the calendar so that every date always falls on the same day (Invariable Calendar) .

If this Invariable Calendar were adopted, then the day that we celebrate as Shabbos now, would not necessarily be a Saturday. In fact, if I understand what they are proposing, then the days that would be Saturdays under our current calendar will move around from year to year and maybe from week to week.

Although I doubt such a reform would ever be adopted, I'm curious what we would do halachically if it were. Would we change the date of Shabbos so that it still always fell on the date that was designated Saturday in the calendar, or would we have to figure out a way to keep Shabbos on the date that would be Shabbos in the current system? (The former might be impossible, because the Invariable Calendar adds a day once a year that has no day of the week. So, if we switched Shabbos to every Saturday in the new system, then at least once a year there would be 8 days between Shabboses.)

The answer to this question depends, I know, on how one rules on this question: [History of Shabbat: how (or) did they count to align with 7th day of creation?

Answer 1

While the Jewish holidays are controlled by the Sanhedrin (though, notably, they are not controlled by non-Jewish governments, and even the Sanhedrin's control is bound by divinely received rules), Sabbath is different in that it always repeats every seventh day, regardless of any calendar. This is one reason given why, in the central prayer of Sabbath/festival/new moons shemoneh esrei, Sabbath is mentioned as directly sanctified by G-d (מקדש השבת), whereas the sanctity of the festivals/new moons is described as corollary to the sanctification of Israel (מקדש ישראל והזמנים).

תני תנא קמיה דרבינא מקדש ישראל והשבת והזמנים אמר ליה אטו שבת ישראל מקדשי ליה והא שבת מקדשא וקיימא אלא אימא מקדש השבת ישראל והזמנים אמר רב יוסף הלכה כרבי וכדתריץ רבינא:

A tanna taught a baraita before Ravina with a slightly different reading: He concludes the blessing with: Who sanctifies the Jewish people, Shabbat, and the seasons. Ravina said to that tanna: Is that to say that the Jewish people sanctify Shabbat? Isn’t Shabbat already sanctified from the six days of Creation? Every seventh day is automatically Shabbat, without the need for any declaration on the part of the Jewish people. Rather, amend it and say as follows: Who sanctifies Shabbat, the Jewish people, and the seasons, as the Jewish people indeed sanctify the New Moon and the Festival days. Rav Yosef said: The halakha with regard to the conclusion of the blessing is in accordance with the opinion of Rabbi Yehuda HaNasi and as the difficulty was resolved by Ravina. (Beitza 17a - hattip: @DonielF)

Regardless, there is no reason to connect Sabbath to Saturday specifically, and, indeed, it is generally assumed that in regions over the secular date line, Sabbath may fall out instead on Friday/Sunday.