About Shabbat

First of all, let's look at the main foundation - the law (in Hebrew, TORAH).
TORAH are called 10 commandments, later a large book was compiled, which consisted of 5 scrolls and it was also called TORAH (Law).
So, what does the main law say about the Sabbath - 10 commandments?

Ex.20:8-11
8 Remember the Sabbath day (day of rest) to keep it holy;
9 work for six days and do all your work,
10 And the seventh day is the Sabbath of the Lord your God: on it you shall do no work, neither you, nor your son, nor your daughter, nor your servant, nor your maidservant, nor your livestock, nor the stranger that is in your dwellings;
11 For in six days the Lord made heaven and earth, the sea, and all that is in them, and rested on the seventh day; wherefore the Lord blessed the Sabbath day and hallowed it.

It does not say about a specific day of the week, does not indicate a reference point, here the principle is given - work for 6 days, and rest on the seventh.
I mean, it's just a day off.
What did the Jews do? They named the days of the week: first, second, third, fourth, fifth, sixth, rest (Shabbat or Saturday).
That's all. Although they didn’t really dwell on it, it was just a day off.
A special relationship to the Sabbath began after the Babylonian captivity. Why?

The reason for sending all of God's people into slavery is not keeping the Sabbath. Specifically:

2Chr.36:19-21
19 And they burned the house of God, and destroyed the wall of Jerusalem, and burned all its halls with fire, and destroyed all its treasures.
20 And he brought those who were left of the sword into Babylon, and they were his servants and his sons, until the reign of the king of Persia,
21 Until, in fulfillment of the word of the Lord, spoken by the mouth of Jeremiah, the earth kept its Sabbaths. In all the days of desolation, she kept the Sabbath until the fulfillment of 70 years.

Shocking information, isn't it? God's people for 70 years into slavery because the earth did not rest. Very strange.
So what is the Sabbath year?

Deut. 15:1 In the seventh year make forgiveness.

So it turns out what's the matter? - Forgiveness.
70 Sabbath years is 7X70=490 years. Reminds me of something?

Mat.18:21,22
21 Then Peter came to Him and said, Lord! how many times shall I forgive my brother who sins against me? up to seven times?
22 Jesus said to him, I do not say to you, up to seven times, but up to seventy times.

You need to forgive 7x70 times. That is, it is about forgiveness.
Let the Jewish slaves go free.
In the year of jubilee, we return the land to those who originally owned it.

After Babylon, there was an attempt to fulfill this commandment:

Jer.34:8-11
8 The word that came to Jeremiah from the Lord after King Zedekiah made a covenant with all the people that were in Jerusalem to declare freedom,
9 that each one should set free his servant and his maidservant, the Jew and the Hebrew woman, so that none of them should keep his brother Jew in bondage.
10 And all the princes and all the people obeyed, which entered into a covenant to release to each his servant and to each his servant to his will, so as not to keep them henceforth in slaves, - and they obeyed and let go;
11 But after that, having considered, they began to take back the slaves and the female slaves, whom they had set free, and forced them to be slaves and handmaidens.

They didn't last long enough. But they came up with such an order for the Sabbath in order to fulfill it without fulfilling it.

Bible Sabbath (more precisely, the day of rest) is about forgiveness and the day off.
And Bible does not indicate which day it is, but simply work for 6 days, and rest on the 7th.