I'd be led to believe you instantly...if it were not for the clear implications of both Genesis and more importantly Exodus 20:11.
In Genesis we see that God SET APART the 7th day--because He "shabbated" on the very first 7th day ever!
Exodus 20:11 expounds on that further by clearly linking the Sabbath day commandment back to creation.
For in six days the LORD made the heavens and the earth, the sea and all that is in them, and rested on the seventh day; therefore the LORD blessed the sabbath day and made it holy."
It doesn't really matter whether or not you believe the above...what is clear is that God clearly set apart the very first 7th day, rested on it ---[how epic is that for the CREATOR to "remember the Sabbath day and keep it holy, resting from His work"] and reminded us of this amazing moment in Exodus 20:11.
God set apart the 7th day from the beginning!
Ever wonder how 6,000+ years later we still have a 7 day week?