Sabbath Was There From the Beginning
Sabbath was established well before the Ten Commandments were written in stone by God.
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.
- Genesis 2:3 (KJV)
The original Hebrew word for "he had rested" is Strong's 7673, meaning Sabbath ("rest").
The Ten Commandments reflect the eternal non-changing character of God, and they are always in place, e.g. we need to keep the commandments, and especially the Sabbath, which is a sign between the Creator and us.
It is a sign between me and the children of Israel for ever: for in six days the LORD made heaven and earth, and on the seventh day he rested, and was refreshed.
- Exodus 31:17 (KJV)
And yes, Christians are the children of Israel.
And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
- Galatians 3:29 (KJV)
Adam and Eve kept the Sabbath, the Jews kept them, the Christians should keep them, and the Sabbath will also be kept in Heaven.