From Wikimedia Commons, the free media repository

A week is a time unit equal to a number of days, now usually seven days. Weeks of between 4 and 20 days have been used historically in various places.

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  Soviet calendar
  12 December 1937
  (Below 12:)
  "Sixth day of the six-day week"
  —————————
  "Election day for the Supreme Soviet of the USSR"

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  Restored Aztec sun stone showing the 20 Days

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  The Gediminas Sceptre, a medieval Lithuanian calendar. Showing 12 months and 9 days in a week