Exodus 20:9 - Ellicott's Commentary On The Whole Bible

Six days shalt thou labour. — The form is certainly imperative; and it has been held that the fourth commandment is “not limited to a mere enactment respecting one day, but prescribes the due distribution of a week, and enforces the six days’ work as much as the seventh day’s rest” (Garden in Smith’s Dictionary of the Bible, vol. iii., p. 1068). But the work on the six days is really rather assumed as what will be than required as what must be; and the intention of the clause is prohibitory rather than mandatory — “thou shalt not work more than six days out of the seven.”