Not a salvation issue
The word rest (sabbath) is not a time sensitive word . Hebrew 4 informs us any time we do not harden our hearts mixing faith in what we do see we have entered the rest of Christ .
Two kind of sabbaths one as a ceremonial shadow the other moral (do not harden ones heart) and refuse to mix faith.
The ceremonial the forth commandment is according to a parable as reasoning law. Two different reasoning are used .One in Exodus 20 the other in Deuteronomy 5. One in creation the other moving out the nation in Deuteronomy .
Exodus 20:11 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.
Deuteronomy 5:15 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.
Parables used to reason are not used with the other moral commandments.
It would seem the rest spoken of in Deuteronomy was the first day of the week, the day he said let there be light guiding them through the red Sea representing children of light, the first day to eat and share the gospel , two days were set aside not to do any regular works.