It was introduced by God in:

Exodus 23: 12  Six days you shall do your work, and on the seventh day you shall rest, that your ox and your donkey may rest, and the son of your female servant and the stranger may be refreshed.

This day was given as a day of rest which was mostly physical. This day was established for men as well as animals, each of them had to rest.

It happens that some Christians today still advocate the practice of the Sabbath in this form, especially because of passages like:

Mark 16:1  Now when the Sabbath was past, Mary Magdalene, Mary the mother of James, and Salome bought spices, that they might come and anoint Him.

So, we can see that these three women respected the Sabbath before going to anoint the body of Jesus.
We can also, in a sense, understand why Jesus had in some occasions respected the Sabbath of the old Covenant, simply by the fact that He still lived in this period of the Law. This time was certainly a  period of overlap with the period of grace, but it was nevertheless still the period of the Law.

This isn't because these three women respected the Sabbath, that they were necessarily right to do so. It is easy to understand and to imagine that these women had observed the Sabbath throughout their lives, and had probably not yet seized the scope of the grace and its accomplishments.

If we try to go deeper in the practice of the Sabbath since the new Covenant, we can see that no one has observed the Sabbath in the way it is explained in Exodus 23:12. Could we conclude that these three women in Mark 16 were wrong ?
No, certainly not.

Certainly the cross was passed, but we were at that time still in this period of overlap, because Jesus had not yet ascended to heaven definitely. Indeed He had to resurrect on the one hand, and come back for 40 days among His Apostles to complete their education, on the other hand.

There is nothing subsequent to the establishment of grace on Earth, which shows that Christians should accomplish or observe the rules of the Sabbath as described in Exodus 23.
There are many passages of the new Covenant which talk about the Sabbath, but these same passages mention it as a reference in the time of the Law, and not as facts. Here are few examples.

Acts 1:12 then they returned to Jerusalem from the mountain called olive trees, which is near Jerusalem, at the distance of a path of Sabbath.

Acts 13:14  Then they returned to Jerusalem from the mount called Olivet, which is near Jerusalem, a Sabbath day’s journey.

Acts 13:27  For those who dwell in Jerusalem, and their rulers, because they did not know Him, nor even the voices of the Prophets which are read every Sabbath, have fulfilled them in condemning Him.

Acts 13:42  So when the Jews went out of the synagogue, the Gentiles begged that these words might be preached to them the next Sabbath.

Acts 13:44  On the next Sabbath almost the whole city came together to hear the word of God.

Acts 15:21  For Moses has had throughout many generations those who preach him in every city, being read in the synagogues every Sabbath.”

Acts 16:13  And on the Sabbath day we went out of the city to the riverside, where prayer was customarily made; and we sat down and spoke to the women who met there.

Acts 18:4  And he reasoned in the synagogue every Sabbath, and persuaded both Jews and Greeks.

We can see that all these references found in the book of Acts, do not refer as acts of physical Sabbath for Christians, but only as references in times. We could simply replace the word Sabbath with the word Saturday. This would give for example:

Acts 13:42  So when the Jews went out of the synagogue, the Gentiles begged that these words might be preached to them the next Sabbath.

Then of course, if the Sabbath is mentioned it means that people respected it.
Absolutely, it's obvious.
But in no case we can read that born again people respected the Sabbath. Paul was speaking to Jews, not yet born again, and still respecting the Sabbath as a physical rest.

We can see also that Luke himself mentioned the term of Sabbath in this verse below:

Acts 13:44  On the next Sabbath almost the whole city came together to hear the word of God.

Can we say that Luc was observing the Sabbath?
Absolutely not.
The term of Sabbath here was only a reference to a day of the week, and was not used in relation to an act that should be respected. In the thought of Luke, the term Sabbath was only used to describe a day of the week, a moment in time, exactly as we may use today the word Saturday.
Being accustomed to all his life to use this term, it is easy to conceive that Luke continued to use it to describe Saturday. Sunday being the first day of the week, then Saturday is the seventh day of the week.

The Bible tells us that the things of the old Covenant is the shadow of the things to come.

As part of the Sabbath, God asked the Jewish people to observe, in addition to one day a week, a whole year of rest every seven years. We can find it in Leviticus 25.
Some of the people  wondered what they would eat during the seventh year if they stopped to harvest in purpose to rest, Leviticus 25: 20.
This is exactly what the Sabbath is all about in the new Covenant: resting in God, getting into this dimension of faith that will produce this peace in Christ, on which I can rest at all times, and not only one day a week.

Then to compensate for this, God said He would bless their crops the sixth year, so they could produce enough for three years, Leviticus 25: 21.
So, we can see that in that Sabbath God provided everything, without exception. He gave enough for the current year, as well as for the year of rest, but also God gave for a third year in order to allow His people to wait for the harvest, following the Sabbath year, to start producing.
Therefore, only one year of harvest had to meet the need of  three years of harvest, and God provided and allowed the people to be fed during that rest of the seventh year, Leviticus 25: 22.

Today the Sabbath is no longer a day or a year where Christians observe a physical rest. The shadow of this image described in Leviticus simply shows us that all Christians have this rest in Christ on a permanent basis. To enter the Sabbath of the new Covenant, it is a matter of thinking, in other words, of faith, and not a matter of doing.

When I am in Christ, then God has already met all my needs.
Philippians 4:19 and my God will supply all your needs according to his riches, with glory in Jesus Christ.

He does not abandon me.
Hebrews 13:5 because God himself said: I will not forsake you, and I won't abandon you point.

There are still a lot of verses that show us that we must enter into this way of thinking that is reinforced by our faith, and which corresponds to the Sabbath of the new Covenant.

The Sabbath is no longer a day or a particular year, but a way of thinking using the faith that I already have received, in order to enter into a permanent rest, where God provides for all my needs.

Bye for now.