Information about the Perpetual 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 10 days dropped. The day after Thursday October 4th, 1582 became Friday October 15th.
 In 1582, Julian calendar year #2 had been used up to October 4th.  Now, Gregorian year calendar #6 would be used
 from October 15th through the end of 1582. Click here for 1582 Calendar

 To keep the dates of annual and seasonal celebrations in sync, Pope Gregory decreed that only century years that
 were divisible by 400 (1600, 2000, 2400, etc) would be 366-day Leap Years. Century years that 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

 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).

 Here is the Year Calendar Sequence as it occurs over any given 28-year period--unless it should happen to be
 interrupted by a Gregorian century year not divisible by 400 »»»
                      »»» 1-2-3-11  6-7-1-9  4-5-6-14  2-3-4-12  7-1-2-10  5-6-7-8  3-4-5-13

 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, 2286 and 2686

 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, 2286 amd 2686)



 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!"

 It has been suggested (by the astronomer John Herschel (1792-1871) among others) that a better approximation to the
 length of the tropical year would be 365 969/4000 days = 365.24225 days.  This would dictate 969 leap years every
 4000 years, rather than the 970 leap years mandated by the Gregorian calendar. This could be achieved by dropping
 one leap year from the Gregorian calendar every 4000 years, which would make years divisible by 4000 non-leap years. 

 To date, this "4000-year rule" has not been officially adopted.              

 Pope Gregory XIII's ruling that dropped the 10 days in 1582 was not a popular move with many people, particularly
 businessmen. Some papal advisors suggested that the Gregorian Calendar be phased in over a given period, but Pope
 Gregory ruled that the revised calendar was to be put in place right away.
     
 Now, let us suppose that that a phase-in period had been ordered. During such a period, the regular observance of
 Leap Year and February 29th would have been put on hold from 1584 to 1620. This period would have included the
 year 1600, which under the Pope's actual ruling was a leap year because it is a century year divisible by 400.
    
             
 A: Year Calendar Sequence (1580-1629) as it actually occured
              
 1580 - 13   1590 -  2   1600 - 14   1610 -  6   1620 - 11    In 1582, Year Calendar-2 is used up thru October 4th,
 1581 -  1   1591 -  3   1601 -  2   1611 -  7   1621 -  6    when Pope Gregory XIII orders the next 10 days dropped.
 1582 - 2/6  1592 - 11   1602 -  3   1612 -  8   1622 -  7
 1583 -  7   1593 -  6   1603 -  4   1613 -  3   1623 -  1
 1584 -  8   1594 -  7   1604 - 12   1614 -  4   1624 -  9
 1585 -  3   1595 -  1   1605 -  7   1615 -  5   1625 -  4    Thus, October 15th becomes the first day after October 4th.
 1586 -  4   1596 -  9   1606 -  1   1616 - 13   1626 -  5    Year Calendar-6 is used from this point to the end of 1582.
 1587 -  5   1597 -  4   1607 -  2   1617 -  1   1627 -  6    The Gregorian Calendar has arrived.
 1588 - 13   1598 -  5   1608 - 10   1618 -  2   1628 - 14
 1589 -  1   1599 -  6   1609 -  5   1619 -  3   1629 -  2


 B: Year Calendar Sequence (1580-1629) if Leap Day February 29 is omitted every 4 years from 1584 to 1620
             
 1580 - 13   1590 -  3   1600 -  6   1610 -  2   1620 -  5    In Sequence B, no days are removed from 1582, and the
 1581 -  1   1591 -  4   1601 -  7   1611 -  3   1621 -  6    observance of Leap Year is put aside from 1584 to 1620.
 1582 -  2   1592 -  5   1602 -  1   1612 -  4   1622 -  7
 1583 -  3   1593 -  6   1603 -  2   1613 -  5   1623 -  1    Beginning 1621, Sequence B is equal with Sequence A above.
 1584 -  4   1594 -  7   1604 -  3   1614 -  6   1624 -  9    Leap Year Observance is restored, beginning 1624, and now
 1585 -  5   1595 -  1   1605 -  4   1615 -  7   1625 -  4    the Gregorian Calendar is fully in place.
 1586 -  6   1596 -  2   1606 -  5   1616 -  1   1626 -  5
 1587 -  7   1597 -  3   1607 -  6   1617 -  2   1627 -  6    Note that 1593, 1594 and 1595 use same Year Calendars
 1588 -  1   1598 -  4   1608 -  7   1618 -  3   1628 - 14    in both Sequences A and B.
 1589 -  2   1599 -  5   1609 -  1   1619 -  4   1629 -  2

 A: The actual reckoning

    January - 31                            July - 31                 Halves - 181 (182) / 184
    February - 28 (29 in leap year)         August - 31               Quarters - 90 (91) / 91 / 92 / 92
    March - 31                              September - 30            
    April - 30                              October - 31
    May - 31                                November - 30
    June - 30                               December - 31


 B: A never adopted alternative reckoning  .. ..  Give this one some thought folks!

    January - 30                            July - 30  (31 in leap year)
    February - 31                           August - 30
    March - 30                              September - 31            
    April - 31                              October - 30
    May - 30                                November - 31             Halves - 183 / 182 (183)
    June - 31                               December - 30             Quarters - 91 / 92 / 91 (92) / 91