The Lord wanted Moses to speak to Israel and to explain why they were to keep the Sabbath, “Surely My Sabbaths you shall keep, for it is a sign between Me and you throughout your generations, that you may know that I am the LORD who sanctifies you” (Exodus 31:12-13).
First, the Sabbath was between ancient Israel and the Lord.
Second, the Sabbath was a sign to let Israel know that the Lord was the One who sanctified them.