Sunday is the first day of the week of the calendar the word of GOD said to remember the seventh and keep it HOLY.Sunset to sunrise is how a day is biblically defined .So sundown Friday would mark the beginning of sabbath, not Sunday morning.
This is how we know morality doesn't come from religion. The bible doesn't say slavery is wrong. The bible has been used to defend slavery. But we know it's wrong. We didn't need the bible to tell us so.