Heb 4:9 So there remains a Sabbath rest for the people of God.
Weren't the Jews keeping the Sabbath as related to the law? Yes, they were keeping the Sabbath to be saved. Why? They had rejected "the Lord of the Sabbath". They weren't resting in God's finished work, the gospel. They were working at meriting salvation. So to them, according to the author of Hebrews, a Sabbath rest still remained open.
10 For the one who has entered His rest has himself also rested from his works, as God did from His.
The one who has accepted the gospel - that is, the one who has entered "God's rest", also does something else. What does the believer, who has already been saved "in Christ", do?
He rests the Sabbath just as God did some 2000 years ago when His work of redemption was finished in Christ Jesus.
Therefore the Sabbath, properly understood, is a sign of justification by faith.
When I cease from my normal everyday labors on the 7th-day Sabbath I am stating, outwardly (as a sign), that I can't add to what God did in Christ Jesus 2000 years ago. Therefore the Sabbath, as connected to the gospel, is anything but legalistic.