Frank's Humble Abode at frankosite2020.com .... formerly known as frankosport.com
The Perpetual Calendar: Introduction Page


Introduction ... 1: Calendar Beginnings

The Julian Calendar was introduced in 45 B.C., and used through 1582 A.D. By that time papal
scholars had estimated that certain annual and seasonal celebrations were off by 10 days.

Pope Gregory XIII issued an order that those 10 days be dropped, all at one time. So, 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. Eleven 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 thru 14) shown below.

Year Calendars ...


Introduction ... 2: 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
        four, 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)             รท       7
   --------------------------------------------------
   Exactly this many WEEKS >>>              =  20,871

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

Frank's Humble Abode at frankosite2020.com .... formerly known as frankosport.com
The Perpetual Calendar: Introduction Page



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