The sabbath day is traditionally from evening on Friday to evening on Saturday.
Sunday is the first day of the week and Saturday is the seventh day of the week.
We know this because the Bible says:
"And there was evening and there was morning, one day." (Genesis 1:5)
From the beginning, each day started when it was dark at night and went to the next day. So each day is from evening to morning (even though in the world people count each day from morning to evening). But God counts each day from evening to morning, just as we are born into darkness but are alive in Christ and will see the dawn of His day.
"Six days you shall labor and do all your work, but the seventh day is a sabbath of the Lord your God...For in six days the Lord made the heavens and the earth, the sea and all that is in them, and rested on the seventh day; therefore the Lord blessed the sabbath day and made it holy." (Exodus 20: 9-11)
So the Sabbath is the 7th day of rest during the week.
From the Bible we also know that on the Sabbath day (Saturday), Jesus' body was in the tomb.
"Now the women who had come with Him out of Galilee followed, and saw the tomb and how His body was laid. Then they returned and prepared spices and perfumes. And on the Sabbath they rested according to the commandment." (Luke 23:55-56)
On the first day of the week (Sunday), Jesus resurrected.
"But on the first day of the week, at early dawn, they came to the tomb bringing the spices which they had prepared. And they found the stone rolled away from the tomb, but when they entered, they did not find the body of the Lord Jesus." (Luke 24:1-3)
And angels said to them, "Why do you seek the living One among the dead? He is not here, but He has risen" (Luke 24: 5-6).
That is why Christians worship God on Sundays. The apostle John called Sundays the Lord's day:
"I was in the Spirit on the Lord’s day, and I heard behind me a loud voice like the sound of a trumpet..." (Revelation 1:10)
And Christians met on the first day of the week to take Communion, worship and listen to God's message, and give to the poor.
"On the first day of the week, when we were gathered together to break bread, Paul began talking to them..." (Acts 20:7).
"Now concerning the collection for the saints, as I directed the churches of Galatia, so do you also. On the first day of every week each one of you is to put aside and save, as he may prosper, so that no collections be made when I come. When I arrive, whomever you may approve, I will send them with letters to carry your gift to Jerusalem..." (1 Corinthians 16:1-3)
For the Jewish people, the Law was given for them to keep the sabbath day holy (meaning doing no work on that day).
When Gentiles (non-Jewish) people became Christians, they wondered how many of the Jewish laws they have to keep (of which there are hundreds in the Law of Moses in the Bible, regarding for example, circumcision, ceremonial washings, religious festivals, sabbaths, clean and unclean foods, etc.). So this question was posed in the early Christian church.
See Acts 15:
https://www.bible.com/bible/100/ACT.15.NASB1995
"But some of the sect of the Pharisees who had believed stood up, saying, 'It is necessary to circumcise them and to direct them to observe the Law of Moses.'
The apostles and the elders came together to look into this matter. After there had been much debate, Peter stood up and said to them, 'Brethren, you know that in the early days God made a choice among you, that by my mouth the Gentiles would hear the word of the gospel and believe. And God, who knows the heart, testified to them giving them the Holy Spirit, just as He also did to us; and He made no distinction between us and them, cleansing their hearts by faith. Now therefore why do you put God to the test by placing upon the neck of the disciples a yoke which neither our fathers nor we have been able to bear? But we believe that we are saved through the grace of the Lord Jesus, in the same way as they also are.'" (Acts 15:5-11)
So the Apostles answered the Gentile Christians this way:
"The apostles and the brethren who are elders, to the brethren in Antioch and Syria and Cilicia who are from the Gentiles, greetings.
Since we have heard that some of our number to whom we gave no instruction have disturbed you with their words, unsettling your souls,
it seemed good to us, having become of one mind, to select men to send to you with our beloved Barnabas and Paul,
men who have risked their lives for the name of our Lord Jesus Christ.
Therefore we have sent Judas and Silas, who themselves will also report the same things by word of mouth.
For it seemed good to the Holy Spirit and to us to lay upon you no greater burden than these essentials:
that you abstain from things sacrificed to idols and from blood and from things strangled and from fornication; if you keep yourselves free from such things, you will do well. Farewell." (Acts 15:23-29)
The Jewish Christians instructed the Gentile Christians that the only things from the Jewish Law they needed to keep are the things above. If you are non-Jewish, then you don't need to keep festivals or sabbaths.
"Therefore no one is to act as your judge in regard to food or drink or in respect to a festival or a new moon or a Sabbath day— things which are a mere shadow of what is to come; but the substance belongs to Christ." (Colossians 2:16-17)
God speaks of the believer's rest in Christ. This is not a day of the week, but our rest in Christ is everyday and our rest is also in heaven.
"Therefore, let us fear if, while a promise remains of entering His rest, any one of you may seem to have come short of it. For indeed we have had good news preached to us, just as they also; but the word they heard did not profit them, because it was not united by faith in those who heard. For we who have believed enter that rest, just as He has said,
'As I swore in My wrath,
They shall not enter My rest,'" (Hebrews 4:1-3)
"Therefore, since it remains for some to enter it, and those who formerly had good news preached to them failed to enter because of disobedience, He again fixes a certain day, 'Today,' saying through David after so long a time just as has been said before,
'Today if you hear His voice,
Do not harden your hearts.'
For if Joshua had given them rest, He would not have spoken of another day after that. So there remains a Sabbath rest for the people of God. For the one who has entered His rest has himself also rested from his works, as God did from His. Therefore let us be diligent to enter that rest, so that no one will fall, through following the same example of disobedience." (Hebrews 4:6-11)
So let us be diligent to enter God's rest.
If we have believed, we should keep God's commandments to love God and love others. If we truly believe, we would keep His commandments and not be disobedient. We would put into practice God's word in our daily lives if we believe.
So if we practice God's word and have faith in Jesus, we will enter His sabbath rest.
For more information, please see:
https://www.gotquestions.org/Sabbath-keeping.html