The problem or confusion that we moderns have with the 7th day/Saturday is partly the shift in calculation from lunar to solar years, and partly historical provincialism.
As Tim alluded to above Rush has written about the role of "fixed Sabbaths" that are set at a specific date. I.e 1, 8, & 15 Nisan (Lev 23:39). Since these dates were set "in stone" then the inter-calendar added dates must have Sabbaths set (calculated) from these days.
The imposition of names for the days of the week is a fairly recent innovation. Since it is the way we do it we assume that it has always been this way. Not so. The Hebrews only namesd the Sabbath, all other days were callled by an ordinal number based on their relationship to the Sabbath. I.e. "the first day of the week" was so callled, because it had no other name! "Monday" had not been invented yet.
The other issuue is the adoption by the rabbi's of a Saturday Sabbath in the early medieval period. The shift from the 7th day to a set day of the week was an accomodation by the rabbi's to the surounding Christian culture.
Here is a simple way to understand the issue.
What day is your birthday? If you answer the "3rd of March" I could then ask "But what day is it? Is it a Monday a Tuesday, what Day is it?" Of course you would think I was simple minded! Everyone "Knows" that a B-day is a DATE, not a DAY.
So it was with the Sabbath. As Lev. 23:39 shows it was a DATE.
Since in our way of reconing the "First Day of the Week" is always "Monday" we fall prey to the common fallacy of historical provincialism when we project into the past our current practice. Add to the mix the confusion in most peoples mind between Biblical Hebrews & early medieval Jewish Rabbi's Plus the added confusion that arises over the shift from numbered days in a lunar year to named days in a solar year... It is no wonder that people get so mixed up.
BTW this entire debate is linked to the subject of the calculation of Easter. Since Easter is celebrated each year on The actual week of the passion of Christ, it wanders in our Solar calendar. That is why Easter is a "Movable Feast".
One intersting fact that is illuminated by a proper (in my opinion) view of the OT Sabbath, is that the OT Sabbath as a Date actualy was celebrated during the course of the year on every Day of the week! Just as your B-day falls in turn on evey day of the week, so the OT Sabbath fell in turn on each "Day" of the week. In this way the rest of God redemed or rescued every day with his rest!