God created the Sabbath for mankind (Mark 2:27 Mark 2:27And he said to them, The sabbath was made for man, and not man for the sabbath:
American King James Version×), and there will yet come a time when all people will keep God's Sabbath.
The Bible speaks of God's Kingdom as being established on earth when Jesus Christ returns to rule as "King of Kings and Lord of Lords" (Revelation 11:15 Revelation 11:15And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever.
American King James Version×; Revelation 19:11-16 Revelation 19:11-16  And I saw heaven opened, and behold a white horse; and he that sat on him was called Faithful and True, and in righteousness he does judge and make war.  His eyes were as a flame of fire, and on his head were many crowns; and he had a name written, that no man knew, but he himself.  And he was clothed with a clothing dipped in blood: and his name is called The Word of God.  And the armies which were in heaven followed him on white horses, clothed in fine linen, white and clean.  And out of his mouth goes a sharp sword, that with it he should smite the nations: and he shall rule them with a rod of iron: and he treads the wine press of the fierceness and wrath of Almighty God.  And he has on his clothing and on his thigh a name written, KING OF KINGS, AND LORD OF LORDS.
American King James Version×). In addition to ruling over sons of God who will be changed to spirit beings and rule with Christ in the Kingdom of God at the time of His second coming, Jesus will rule as Head of a literal government over physical nations on earth (Psalms 22:27-28 Psalms 22:27-28  All the ends of the world shall remember and turn to the LORD: and all the kindreds of the nations shall worship before you.  For the kingdom is the LORD's: and he is the governor among the nations.
American King James Version×; Psalms 72:1-11 Psalms 72:1-11  Give the king your judgments, O God, and your righteousness to the king's son.  He shall judge your people with righteousness, and your poor with judgment.  The mountains shall bring peace to the people, and the little hills, by righteousness.  He shall judge the poor of the people, he shall save the children of the needy, and shall break in pieces the oppressor.  They shall fear you as long as the sun and moon endure, throughout all generations.  He shall come down like rain on the mown grass: as showers that water the earth.  In his days shall the righteous flourish; and abundance of peace so long as the moon endures.  He shall have dominion also from sea to sea, and from the river to the ends of the earth.  They that dwell in the wilderness shall bow before him; and his enemies shall lick the dust.  The kings of Tarshish and of the isles shall bring presents: the kings of Sheba and Seba shall offer gifts.  Yes, all kings shall fall down before him: all nations shall serve him.
American King James Version×; Daniel 2:34-35 Daniel 2:34-35  You saw till that a stone was cut out without hands, which smote the image on his feet that were of iron and clay, and broke them to pieces.  Then was the iron, the clay, the brass, the silver, and the gold, broken to pieces together, and became like the chaff of the summer threshing floors; and the wind carried them away, that no place was found for them: and the stone that smote the image became a great mountain, and filled the whole earth.
American King James Version×; Zechariah 14:8-9 Zechariah 14:8-9  And it shall be in that day, that living waters shall go out from Jerusalem; half of them toward the former sea, and half of them toward the hinder sea: in summer and in winter shall it be.  And the LORD shall be king over all the earth: in that day shall there be one LORD, and his name one.
American King James Version×).
At that time, all nations will be ruled by God's laws (Jeremiah 31:31-34 Jeremiah 31:31-34  Behold, the days come, said the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah:
 Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they broke, although I was an husband to them, said the LORD:
 But this shall be the covenant that I will make with the house of Israel; After those days, said the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people.
 And they shall teach no more every man his neighbor, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them to the greatest of them, said the LORD: for I will forgive their iniquity, and I will remember their sin no more.
American King James Version×; Micah 4:2 Micah 4:2And many nations shall come, and say, Come, and let us go up to the mountain of the LORD, and to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for the law shall go forth of Zion, and the word of the LORD from Jerusalem.
American King James Version×; Isaiah 2:2-3 Isaiah 2:2-3  And it shall come to pass in the last days, that the mountain of the LORD's house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow to it.  And many people shall go and say, Come you, and let us go up to the mountain of the LORD, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem.
American King James Version×; Hebrews 8:7-13 Hebrews 8:7-13  For if that first covenant had been faultless, then should no place have been sought for the second.  For finding fault with them, he said, Behold, the days come, said the Lord, when I will make a new covenant with the house of Israel and with the house of Judah:  Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, said the Lord.  For this is the covenant that I will make with the house of Israel after those days, said the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:  And they shall not teach every man his neighbor, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest.  For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more.  In that he said, A new covenant, he has made the first old. Now that which decays and waxes old is ready to vanish away.
American King James Version×).
All of humanity will then learn and keep God's Sabbath: "'From one New Moon to another and from one Sabbath to another, all mankind will come and bow down before me,' says the Lord" (Isaiah 66:23 Isaiah 66:23And it shall come to pass, that from one new moon to another, and from one sabbath to another, shall all flesh come to worship before me, said the LORD.
American King James Version×, New International Version).
The Sabbath will be kept by gentiles and those who were never a part of the physical nation of Israel. "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 . . .
"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'" (Isaiah 56:1-2 Isaiah 56:1-2  Thus said the LORD, Keep you judgment, and do justice: for my salvation is near to come, and my righteousness to be revealed.
 Blessed is the man that does this, and the son of man that lays hold on it; that keeps the sabbath from polluting it, and keeps his hand from doing any evil.
American King James Version×, Isaiah 56:4-7 Isaiah 56:4-7  For thus said the LORD to the eunuchs that keep my sabbaths, and choose the things that please me, and take hold of my covenant;  Even to them will I give in my house and within my walls a place and a name better than of sons and of daughters: I will give them an everlasting name, that shall not be cut off.  Also the sons of the stranger, that join themselves to the LORD, to serve him, and to love the name of the LORD, to be his servants, every one that keeps the sabbath from polluting it, and takes hold of my covenant;  Even them will I bring to my holy mountain, and make them joyful in my house of prayer: their burnt offerings and their sacrifices shall be accepted on my altar; for my house shall be called an house of prayer for all people.
American King James Version×).
Sabbath observance is specifically mentioned as a part of the covenant God will make with all peoples then, and their worship of Him. It will be an important part of Jesus Christ's coming reign on earth!