ASBI Pocket Beit Midrash: Aruch HaShulchan Orach Chaim 242:1

The Holy Sabbath is the great sign between the Holy Blessed One and God's people, Israel, as it says "for it is a sign between me and you so that you know that I am the Lord who sanctifies you." That is to say that even though Shabbat is a commemoration of creation, "for in six days did the Lord make Heaven and Earth and on the seventh day it was Shabbat and He rested" and therefore "and God blessed the seventh day and sanctified it for on it He rested" etc. as is written regarding creation. And if that is so, it is relevant for all of God's creations. And Shabbat is not comparable to the holidays, which are a commemoration of the exodus from Egypt and the other nations have no relevance or connection to them for the other nations did not leave Egypt! However, everyone was created as a result of creation. And nonetheless, the Holy Blessed One did not give the sanctity of Shabbat to anyone other than Israel. And this is the meaning of "to know that I am the Lord who makes you holy" that is to say that you are holy alongside me, as it says, "you shall be holy [for I...am holy]" and therefore I have given the sanctity of Shabbat to you. For Shabbat and Israel are the two end purposes of creation. (And this is the meaning of the line from the siddur "and he has not given Shabbat to the nations of the earth nor apportioned it..." That is to say: even though they apparently have a relevant connection to Shabbat, [God did not give Shabbat to them]. And this is what is meant at the opening chapter of Massechet Shabbat, "I have a special gift in my storehouse and its name is Shabbat, go and tell Israel etc." That is to say, even though it could have been given to all creation.