In the Old Covenant which God made with Israel at Sinai, the people of Israel were commanded to observe the seventh day of the week.
The seventh day was called the Sabbath.
Sabbath comes from a Hebrew word meaning cease or rest.
Exodus 20:8 Remember the sabbath day, to keep it holy. 9 Six days shalt thou labour, and do all thy work: 10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: 11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
The days began and ended at sundown (approx. 6pm).
The Jewish days are the same today.
This means that the Sabbath begins at 6pm on Friday and ends at 6pm on Saturday.
The Jews were to observe it by not doing any work, including lighting fires and cooking.
In the New Covenant in Christ we are freed from observing the Jewish Sabbath along with the rest of the ordinances of the Law of Moses.
Romans 6:14… for ye are not under the law, but under grace.
Galatians 3:13 Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree:
Only one observance is required of Christians and that is to meet together to share bread and wine in remembrance of Jesus’ sacrifice.
1 Corinthians 11:23 For I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread: 24 And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. 25 After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. 26 For as often as ye eat this bread, and drink this cup, ye do shew the Lord’s death till he come.
No day is specified for this observance. We are to do it ‘often’.
The early Christians found it convenient to meet on the first day of the week (Sunday).
Acts 20:7 And upon the first day of the week, when the disciples came together to break bread…
This has been the tradition adopted by Christians.
Some Christians insist on calling Sunday a ‘Sabbath’ but it clearly isn’t.
Neither do they observe it in the way that the Jews were commanded.
Sunday and the Sabbath: https://www.thisisyourbible.com/library/show/176
I hope you find this helpful.