Week

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.

• 
  Soviet calendar
  12 December 1937
  (Below 12:)
  "Sixth day of the six-day week"
  —————————
  "Election day for the Supreme Soviet of the USSR"
• 
  Restored Aztec sun stone showing the 20 Days
• 
  The Gediminas Sceptre, a medieval Lithuanian calendar. Showing 12 months and 9 days in a week