"The Sabbath is not indefinite, any seventh day after six labor.
The law
from Sinai (Exodus 20: 8-11) makes it as definite as language can make
it.
---The events that gave it birth (Thus the heavens and the earth were finished...Genesis 2: 1-3) confine it to the
definite seventh day.
The 6,240 Sabbath miracles in the wilderness,
three each week for 
forty years-a double supply of manna on the sixth
day, the preservation of the sixth-day manna on the seventh day, and
none on the seventh day (Exodus 16)- show that it is one particular day,
and not simply a proportion of time.
*To claim otherwise would be like
claiming that Washington's Birthday or Independence Day was only a 365th
part of a year, and might be celebrated on any other day as well as the
day upon which it occurred." Uriah Smith