If the spirit of the law is simply "one day out of seven," how come the priests were not commanded to rest one of the other days? They took turns serving in the temple for a certain term. During that term they would work harder on the Sabbath than any other day, yet there is no law telling them to rest Sunday or Monday or every seventh day. If God had meant "every seventh day after six days of work," we'd find some kind of law for the priests.
Additionally, the Sabbath is the only day God blessed. If the Sabbath were any seventh day, there would be no need for God to bless a specific day. Also, He would have told the Israelites to keep whatever day was convenient for them, rather than telling them a specific day to keep.
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