The Sabbath was a sign between God and the children of Israel, not God and the Gentiles. There is no mention of a Sabbath observance from Adam to Moses. It is incredible to think that the Sabbath could have existed as an institution for 2,500 years without a single mention of it. It was imposed only on Israel as a part of the law. Under the new covenant it is a picture of the perfect rest we have in Jesus.