But the Gregorian calendar is accepted to be roughly three years inaccurate. That being the case, the succession of seventh days would not end up on "Saturdays".
I believe your comprehension skills to be somewhat lacking in this instance. Again here we go. God confirmed the 7th day in the wildnerness by withholding manna. Jews have observed Sabbath on EVERY 7th day since. It now happens to coincide with the gregorian Saturday. Can it be spelled out any more clearly?