The actual text of the commandment, from both places where it is derived:
Exodus 20: 8-11
Remember the sabbath day, to keep it holy. Six days shalt thou labour, and do all thy work: 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:* 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.***
Deuteronomy 5: 12-15:
Keep the sabbath day to sanctify it, as the LORD thy God hath commanded thee. Six days thou shalt labour, and do all thy work: 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, nor thy manservant, nor thy maidservant, nor thine ox, nor thine {donkey}, nor any of thy cattle, nor thy stranger that is within thy gates; that thy manservant and thy maidservant may rest as well as thou. And remember that thou wast a servant in the land of Egypt, and that the LORD thy God brought thee out thence through a mighty hand and by a stretched out arm: therefore the LORD thy God commanded thee to keep the sabbath day.
You can see in the text the literal structure of the commandment: six days shalt thou labor, but the seventh is holy; for in six days God made the Heavens and the earth, but in the seventh He rested. This pattern repeats in a number of places throughout Exodus, Leviticus, and Deuteronomy, especially in God’s giving of the manna in the wilderness. I don’t think there’s any mistaking the intention: Sabbath is to come at the conclusion of the week, on the last day thereof, just as God created from the first day to the sixth and rested upon the seventh.
I really do think you will find conservative Adventists who will argue that our Saturday is the exact seven-day anniversary of the day upon which the Lord rested: that, if you could count backwards by weeks far enough, you would find God resting on the exact day corresponding to our Saturday. You don’t have to agree with that but it is what many Adventists believe.
And Adventists, by the way, generally would not say that keeping Sabbath is a matter of salvation for Christians currently living. The ones I knew, while believing that other Christians were lax in their study of the Scriptures, ignorant of the commandments and how they represent God’s perfect Moral Law–right down to the 4th Commandment specifying the Sabbath–but they feel that God winks at the ‘times of ignorance’. In the actual ‘time of Jacob’s Troubles’ (their parallel to the tribulation of other Christians), it will become increasingly clear that the Roman Catholic Church is Babylon and that keeping Her commandments instead of God’s Law represents the ‘mark’ of Antichrist. At that point, Sabbath-keeping will become an absolute issue of salvation. Before then, it rarely would be so. I might be an exception in Adventist eyes, and perhaps even you, since we have studied the issue of the Law and have accepted man’s traditions instead of God’s Law. But for most Christians, what the RCC dubs ‘invincible ignorance’ applies (not that Adventists would actually use that precise theological phrase).