Exo 31:16 And the sons of Israel shall observe the Sabbath, to do the Sabbath for their generations; it is a never ending covenant. 31:13 And you speak to the sons of Israel, saying, You shall surely keep my sabbaths; for it is a sign between me and you for your generation; to know that I am Jehovah your sanctifier.
It is true that God said the Sabbath is a sign between Him and the Israelis. Jesus said Mar 2:27 …The Sabbath was made for man, and not man for the Sabbath. – Jesus did not say that the Sabbath celebration belongs to all people, including converts. Nor did the apostles confirm this kind of doctrine. However, it is in force as part of ten commands “to keep it holy,” and the rest day is somewhat sabbatical, but does not include the pedantic commandments of the Sabbath. For example, the Sabbath rules stipulated how many steps a man could walk on the Sabbath. When the disciples returned to Jerusalem from the Mount of Olives, it was told to be “at sabbatical distance”. Act 1:12 Then they returned to Jerusalem from the Mount of Olives, which is near Jerusalem, a Sabbath day’s journey away. – Jesus also said that he was the Lord of the Sabbath!
Thus, the Sabbath was given to the Israelis. OT is full of affirmations for this. This is a typical example (Mar 2:27) where the doctrine is constructed on the basis of one verse and overlooking many others.