Well, let us ask the Bible and see what council and advice it will give to us as to what the God of the Bible is saying about the questions and advising us humans of the best ways to live.
What do I do; Exodus 20:9 “Six days shalt thou labour, and do all thy work:” So Sabbath is a “Work Free Day.”
Which Day; Exodus 20:10 “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:” So Everybody gets a day off, if they are within your boundaries, Your Employees, everybody gets a day off as well.
Why Should I; 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.” Creator Worship or respect for the Creator.
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.” Redeemer worship as once I was “Lost in Sin (Egypt) but no I am found, was Blind but now I see.”