Question:Now if the Sabbath was only for the nation of Israel, why does it says in Isaiah 56:6-8 that God wanted the strangers (non-Jews) to keep and observe His Sabbath along with the covenant besides the nation of Israel. Now think about it, if he wanted people outside the nation of Israel to observe His commandments during the Old Testament days, then it is no surprise that God wants us to examine it also.
"Thus says the LORD: "Keep justice, and do righteousness, for My salvation is about to come, and My righteousness to be revealed. Blessed is the man who does this, and the son of man who lays hold on it; who keeps from defiling the Sabbath, and keeps his hand from doing any evil." Do not let the son of the foreigner who has joined himself to the LORD speak, saying, "The LORD has utterly separated me from His people"; nor let the eunuch say, "Here I am, a dry tree." For thus says the LORD: "To the eunuchs who keep My Sabbaths, and choose what pleases Me, and hold fast My covenant, even to them I will give in My house and within My walls a place and a name better than that of sons and daughters; I will give them an everlasting name that shall not be cut off. Also the sons of the foreigner who join themselves to the LORD, to serve Him, and to love the name of the LORD, to be His servants - everyone who keeps from defiling the Sabbath, and holds fast My covenant - even them I will bring to My holy mountain, and make them joyful in My house of prayer. Their burnt offerings and their sacrifices will be accepted on My altar; for My house shall be called a house of prayer for all nations." The Lord GOD, who gathers the outcasts of Israel, says, "Yet I will gather to him others besides those who are gathered to him" (Isaiah 56:1-8).
The covenant brought to Israel by Moses allowed people from other nations to enter into the covenant. For instance it is mentioned, "And in every province and city, wherever the king's command and decree came, the Jews had joy and gladness, a feast and a holiday. Then many of the people of the land became Jews, because fear of the Jews fell upon them" (Esther 8:17). These are the people who are called proselytes in the New Testament. They had partial rights under the law. For instance, they were not allowed to serve in the tabernacle or temple. Levites "shall be joined with you and attend to the needs of the tabernacle of meeting, for all the work of the tabernacle; but an outsider shall not come near you" (Numbers 18:4). They could keep the Passover, but only if they followed the same rules as the Israelites. "And if a stranger dwells among you, and would keep the LORD'S Passover, he must do so according to the rite of the Passover and according to its ceremony; you shall have one ordinance, both for the stranger and the native of the land" (Numbers 9:14). That meant that the foreign born had to enter into the covenant first. "And when a stranger dwells with you and wants to keep the Passover to the LORD, let all his males be circumcised, and then let him come near and keep it; and he shall be as a native of the land. For no uncircumcised person shall eat it. One law shall be for the native-born and for the stranger who dwells among you" (Exodus 12:48-49).
The passage in Isaiah is an assurance to people who could not have full access to the Temple that they could still be acceptable to God. This is why eunuchs are also listed because they too could not enter the temple.
This passage does not prove that God wanted all people to observe the Sabbath. Nor does it prove that the Sabbath was an ordinance carried over into the New Testament.