It was a sign that we are obedient to God, also just like circumcision. Unless you can tell me why the precise timing of the Sabbath is a moral law and not merely ceremonial, I'll have to conclude that both the Sabbath and circumcision are both moral laws, and also both ceremonial laws. I've already explained. So, even though circumcision was a sign of God's true people, the ceremonial aspects of it have been done away, and true circumcision is now in our hearts. Similarly, even though the Sabbath was a sign of God's covenant with his people, the ceremonial aspects have been done away, and the true Sabbath is in our hearts.
Today there is no legalistic Sabbath any more, only the Sabbath's worship of our hearts.
Another point brought up in the IRC chat and not answered by any SDA was that the Sabbath was a sign between God and Israel, NOT between God and humanity. So the ceremonial Sabbath applied to Israel, not humanity. Since Christainity goes out to the whole world today, the ceremonial aspects of circumcision and the Sabbath do not apply to everyone, but the moral aspects do - the internal love and worship of God and neighbour that was taught by Jesus. Finally, I'd like to deal with the issue of the earliest historical record for Sunday usage. But let no one that is at variance with his fellow come together with you, until they be reconciled, that your sacrifice may not be profaned.