But the seventh day is the sabbath of the LORD thy God:
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.
Exodus 20:8,10,11
"The Sabbath command refers back to the creation week, and it is in the historical setting of that week that the phrase “the seventh day” must be understood.
The Sabbath is based upon the events of the creation week. (Gen. 1:1-2:3).
Exodus 20:8,10,11
All other time cycles are tied to the movements of celestial objects—the month is measured by the phases of the moon, the year is the time it takes the earth to orbit the sun—but the Sabbath exists only in reference to the creation week.
Now, in reference to that first week we ask:
Q: Was the Sabbath simply one day among the seven days in that first week?
A: No, it was the seventh and last day of that original week.
Q: Why would the Sabbath become less specific in succeeding weeks and years and centuries?"
F.D. Nichols
