The Collaborative International Dictionary of English v.0.48:

Week \Week\, n. [OE. weke, wike, woke, wuke AS. weocu, wicu,
   wucu; akin to OS. wika, OFries. wike, D. week, G. woche, OHG.
   wohha, wehha, Icel. vika, Sw. vecka, Dan. uge, Goth. wik?,
   probably originally meaning, a succession or change, and akin
   to G. wechsel change, L. vicis turn, alternation, and E.
   weak. Cf. Weak.]
   A period of seven days, usually that reckoned from one
   Sabbath or Sunday to the next.
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         I fast twice in the week.                --Luke xviii.
                                                  12.
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   Note: Although it [the week] did not enter into the calendar
         of the Greeks, and was not introduced at Rome till
         after the reign of Theodesius, it has been employed
         from time immemorial in almost all Eastern countries.
         --Encyc. Brit.
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   Feast of Weeks. See Pentecost, 1.

   Prophetic week, a week of years, or seven years. --Dan. ix.
      24.

   Week day. See under Day.
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