Introduction of the Gregorian Calendar
  The Julian Calendar was introduced in 45 B.C., and used through 1582 A.D., when it was estimated that certain annual
  and seasonal celebrations were off by 10 days.
         
  Pope Gregory XIII ordered those 10 days to be dropped. The day after Thursday October 4th, 1582 became Friday October
  15th. In 1582, Julian Year Calendar #2 was used up to October 4th.  Beginning October 15th through the end of 1582,
  Gregorian Year Calendar #6 would be used.    Click here for 1582 Calendar (opens in new window or tab).

  To keep the dates of annual and seasonal celebrations in sync, Pope Gregory decreed that only century years which
  were divisible by 400 (1600, 2000, 2400, etc) would be 366-day Leap Years. Century years which were not divisible
  by 400 (1700, 1800, 1900, 2100, 2200, 2300, etc) would only be regular 365-day years.

  The change was adopted almost at once by most countries. However, the British Empire and American Colonies did not
  change over until September 2nd, 1752. 11 days were dropped, making the next day September 14th.
  Click here for 1752 Calendar (opens in new window or tab).

  Julian and Gregorian Systems both use 14 different Year Calendars -- seven 365-day regular year calendars
  (1-2-3-4-5-6-7) and seven 366-day leap year calendars (8-9-10-11-12-13-14). You can view each of these
  year calendars by clicking on any of the blue-colored numbers (01 to 14) located below this frame.



Back to Square Every 400 Years
   If you were to look up certain specific dates in the Gregorian Calendar era, such as these ...

      A: The 25th of September in 1965, 2365 and 2765.

      B: The 15th of October in 1681, 2081, 2481 and 2881

      C: The 4th of July in 1836, 2236 and 2636

   ... you would find that the day-of-the-week and specific year calendar used are always the same, as follows ...

      A: September 25th ... Saturday / Year Calendar # 6  (1965, 2365 and 2765)

      B: October 15th ... Wednesday / Year Calendar # 4   (1681, 2081, 2481 and 2881)

      C: July 4th ... Monday / Year Calendar # 13         (1836, 2236 and 2636)


   Why do the Gregorian Calendar dates "come back to square" so to speak every 400 years?  One simple answer is this --

   Saturday January 1st, 1600 thru Friday December 31st, 1999

      300 standard non-leap years x 365 days   = 109,500

      Years 1700, 1800 and 1900 (365 days x 3) +   1,095   Century years not divisible by 400

      96 standard leap years x 366 days        +  35,136   Years XX04 thru XX96, divisible by 4, in each century
 
      Century Leap Year, 1600                  +     366   A century year divisible by 400
      --------------------------------------------------
      Total number of days                     = 146,097
      divide by 7 (days in a week)                    7
      --------------------------------------------------
      Exactly this many WEEKS >>>              =  20,871

   And January 1st, 2000 is on--SATURDAY!    You can "guess the rest!"

SEE ALSO:
Was it really necessary to drop those 10 days in 1582?          Other Facts and Info          External Links


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