Concerning the "true" Sabbath day, it may not be Saturday or Sunday, neither one.
For all we know it could be Wednesday, or Thursday, but what's the difference when
we are supposed to be edifying each other, and it turns into an argument about what
day of the week this or that should be observed, when none of us actually know.
The Origins of the Seven-Day Week, Eviatar Zerubavel
The Seven Day Circle: The History and Meaning of the Week, Zerubavel
"Days, months, and years were given to us by nature, but we invented the week
for ourselves. There is nothing inevitable about a seven-day cycle, or about any
other kind of week; it represents an arbitrary rhythm imposed on our activities,
unrelated to anything in the natural order. But where the week exists—
and there have been many cultures where it doesn't—it is so deeply embedded
in our experience that we hardly ever question its rightness, or think of it as an
artificial convention; for most of us it is a matter of 'second nature.'
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-Astronomy observence
"The human race never lost the septenary [seven day] sequence of week days and
that the Sabbath of these latter times comes down to us from Adam, though the ages,
without a single lapse."—Dr. Totten, professor of astronomy at Yale University.
"Seven has been the ancient and honored number among the nations of the earth.
They have measured their time by weeks from the beginning. The origin of this was
the Sabbath of God, as Moses has given the reasons for it in his writings."
—Dr. Lyman Coleman.
"By calculating the eclipses, it can be proven that no time has been lost and the
creation days were seven, divided into 24 hours each."—Dr. Hinkley,
The Watchman, July 1926 [Hinkley was a well-known astronomer].
"There has been no change in our calendar in past centuries that has affected in
any way the cycle of the week."—James Robertson, Director American Ephemeris,
Navy Department, U.S. Naval Observatory, Washington, D.C., March 12, 1932.
"It can be said with assurance that not a day has been lost since Creation, and all
the calendar changes notwithstanding, there has been no break in the weekly cycle."
—Dr. Frank Jeffries, Fellow of the Royal Astronomical Society and Research
Director of the Royal Observatory, Greenwich, England.
There is no question about which day the seventh day of the week is.
The weekly cycle
the break between the shift from the Julian to the Gregorian calendar?
There was a well known break of 10 days.
In 1582, that Thursday the 4th was followed by Friday the 15th.
The weekly cycle was not interrupted. Now take a look at 1752, Wednesday
the 2nd was followed by Thursday the 14th. Again, the calendar had to be
adjusted to correct it to the seasons but the weekly cycle remained unchanged.
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The seventh day was set apart and made holy at creation...
Exo 20:11 For in six days the LORD made heaven and earth, the sea, and all that in them is,
and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
God in the wilderness (Jesus Christ) revealed the seventh day to Israel for forty YEARS.
Every meal they were reminded which day was the Sabbath or not.
At the time of Christ, He and his apostles keep time by Gods calender,
but this was changed by man made tradations after Christ died.
.... How sad. 
