When God instituted the first day in Genesis, the darkness (evening) came before the light (morning).
Genesis 1:5 And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
The day began at sundown (approximately 6.00pm) and ended the following day at sundown. Days are still observed by the Jews in this way. WThus, the seventh day which God designated the ‘Sabbath’ begins at approximately 6.00pm on Friday and finishes at approximately 6.00 pm on Saturday.
Sabbath observance was required under the Old Covenant made with Israel at Sinai. It was one of the ten commandments written on stone tablets.
Exodus 20:8 "Remember the Sabbath day, to keep it holy. 9 Six days you shall labor and do all your work, 10 but the seventh day is the Sabbath of the LORD your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your cattle, nor your stranger who is within your gates. 11 For in six days the LORD made the heavens and the earth, the sea, and all that is in them, and rested the seventh day. Therefore the LORD blessed the Sabbath day and hallowed it.
Those who believe the gospel and are baptised into Jesus Christ are not required to observe the Old Covenant and the Law given through Moses. In Christ we come under the New Covenant.
The New Covenant was promised by Jeremiah:
Jeremiah 31:31 "Behold, the days are coming, says the LORD, when I will make a new covenant with the house of Israel and with the house of Judah
The writer to the Hebrews explains:
Hebrews 8:8 Because finding fault with them, He says: "Behold, the days are coming, says the LORD, when I will make a new covenant with the house of Israel and with the house of Judah...
Hebrews 8:13 In that He says, "A new covenant," He has made the first obsolete. Now what is becoming obsolete and growing old is ready to vanish away.
Hebrews 12:24 to Jesus the Mediator of the new covenant, and to the blood of sprinkling that speaks better things than that of Abel.
In Christ, we are to keep his commandments. His commandments do not include Sabbath observance - we are not under the Law of Moses.
Galatians 3:24 Therefore the law was our tutor to bring us to Christ, that we might be justified by faith. 25 But after faith has come, we are no longer under a tutor.
Colossians 2:16 So let no one judge you in food or in drink, or regarding a festival or a new moon or sabbaths, 17 which are a shadow of things to come, but the substance is of Christ.
I hope this helps,