The Sabbath was part of the ceremonial law, and the ceremonial law is a shadow of a reality to come. When the foreshadowed reality comes, then the ceremony is not necessary any more. There was ceremonial law, moral law, and civil law in the Old Testament:
“‘Thou shalt not covet’ is a moral precept; ‘Thou shalt circumcise every male on the eighth day’ is a symbolical precept. The rest of the Sabbath we consider no longer binding as an observance, now that the hope of our eternal rest has been revealed.” (St. Augustine)
HEB 4:9 There remains, then, a Sabbath-rest for the people of God. HEB 4:11 Let us, therefore, make every effort to enter that rest.
So in a real sense, the fulfillment of the Sabbath law is to enter God’s rest, the Sabbath is fulfilled in Christ:
COL 2:16-17 Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day. These are a shadow of the things that were to come; the reality, however, is found in Christ.
Heb. 4:3 “Now we who have believed enter that rest.”
Now if “rest” is present rest for God, then God is still at rest, it is still the seventh day on which God is resting, and if we believe in Jesus, we also enter that rest, every day is Sunday, in a sense. The Sabbath is fulfilled in the rest that Jesus gives, that is what the ceremony pointed to, and thus we may “keep the Sabbath” every day, just as we may “keep the Passover” every time we have communion, which is what the Passover foreshadowed:
1CO 5:7 For Christ, our Passover lamb, has been sacrificed. Therefore let us keep the Festival, not with the old yeast, the yeast of malice and wickedness, but with bread without yeast, the bread of sincerity and truth.
Yet there is a principle established by the days of creation, to rest one day out of seven, which was not ceremonial, “The Sabbath was made for man,” and this time set apart for resting and seeking God is still needed.
Yet the ceremonial aspect is over, this is not a rule such as Israel had under Moses, where someone gathering sticks on the Sabbath was to be stoned (Num. 15:32-36).
Just as the land was to rest for a reason, so that the ground could be replenished, so man is to rest on a regular basis, for a similar reason, and also for a greater one, to set aside time seeking God:
Isaiah 58:13-14 “If you keep your feet from breaking the Sabbath and from doing as you please on my holy day, if you call the Sabbath a delight and the Lord’s holy day honorable, and if you honor it by not going your own way and not doing as you please or speaking idle words, then you will find your joy in the Lord, and I will cause you to ride on the heights of the land and to feast on the inheritance of your father Jacob.” The mouth of the Lord has spoken.