Exodus 20:8-11 reads:
8 “Remember the Sabbath day, to keep it holy. 9 Six days you shall labor, and do all your work, 10 but the seventh day is a Sabbath to the Lord your God. On it you shall not do any work, you, or your son, or your daughter, your male servant, or your female servant, or your livestock, or the sojourner who is within your gates. 11 For in six days the Lord made heaven and 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.
Lewis Sperry Chafer in his book "Grace" suggests the Sabbath Day command is a test as to whether we obey the teaching of grace or the Torah. Many of us still hold this viewpoint, namely that only 9 of the 10 commands apply to the church or the New Covenant era. So which choice BEST approaches your perspective. It doesn't need to be a perfect fit.
We are talking, BTW, or attempting to implement the command. If we cannot document when the first Sabbath actually was, that is -- by the creator of this post's ruling -- a worthless point.