tn This is an adverbial accusative of time, indicating that work may be done for six days out of the week.
tn The form is a Niphal imperfect; it has the nuance of permission in this sentence, for the sentence is simply saying that the six days are work days—that is when work may be done.
tn The expression is שַׁבַּת שַׁבָּתוֹן (shabbat shabbaton), “a Sabbath of entire rest,” or better, “a sabbath of complete desisting” (U. Cassuto, Exodus, 404). The second noun, the modifying genitive, is an abstract noun. The repetition provides the superlative idea that complete rest is the order of the day.