The Fixed Day

"The Sabbath of the commandment was of universal application to commemorate the work of creation; that it was "on a fixed seventh day;" and that it was "on a fixed day of the week." 

And, when the Sabbath was on "a fixed seventh day," and "on a fixed day of the week," that is plainly to admit nothing else than that the creation Sabbath was on a fixed seventh day of the week." 

A.T. Jones

Thus the heavens and the earth were finished, and all the host of

them. And on the seventh day God ended His work which He had made; and He rested on the seventh day from all His work which He had made. And God blessed the seventh day, and sanctified it: because that in it He had rested from all His work which God created and made. Remember the sabbath day, to keep it holy. 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.
Genesis 2:1-3/Exodus 20:8,11