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<img height='225' src='http://vaticanstamps.org/VPS/images/296/715.jpg' >
<img height='225' src='http://vaticanstamps.org/VPS/images/296/716.jpg' >
<img height='225' src='http://vaticanstamps.org/VPS/images/296/717.jpg' > 
400th Anniversary of the Gregorian Calendar Scott 715-717 (1982)
 
<img height='325' src='http://vaticanstamps.org/VPS/images/296/717a.jpg' > 
400th Anniversary of the Gregorian Calendar 
Souvenir Sheet, Scott 717a (1982)</center>
 
On February 24, 1582, the papal bull, ”Inter gravissimas” (“Among the most serious”) was issued by Pope Gregory XIII, establishing the Gregorian Calendar.
 
The three stamps shown at the top of the article, along with the souvenir sheet containing the three stamps, were issued in 1982 to commemorate the 400th anniversary of the reform of the calendar. The stamps are based on the bas-relief sculpted on the tomb of Pope Gregory XIII found at Saint Peter’s Basilica:
 
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<img height='375' src='https://upload.wikimedia.org/wikipedia/commons/8/89/Gregorianscher_Kalender_Petersdom.jpg'> 
Tomb of Pope Gregory XIII, Saint Peter's Basilica, Rome 
Photo by Rsuessbr 
From Wikimedia Commons, used under the terms of the Creative 
Commons Attribution-Share Alike 3.0 Unported license</center>
 
One may ask: why change the calendar from the existing Julian Calendar? The reason for the change lies in the fact that the Julian Calendar counted one year (determined to be the time between vernal equinoxes) as 365.25 days when, in fact, it was 11 minutes shorter. As a result, since the scheduling of Easter was tied to the vernal equinox, the holiday was slowly moving earlier each year, due to accumulated differences in time.
 
In 1545, the Council of Trent authorized Pope Paul III to reform the calendar to allow for more consistent and accurate scheduling of the feast of Easter. In 1577, a Compendium was sent to expert mathematicians soliciting comments. Two prominent scholars whose recommendations were adopted were the Calabrian doctor Aloysius Lilius (or Lilio) and the German mathematician Christopher Clavius (who is pictured on a Vatican City stamp at the bottom of this article). The calendar changes involved establishing a new average length of the year (altered from 365.25 to 365.2425 days) and maintaining the practice of altering the calendar to 366 days each year that is divisible by 4 (“leap years”). It also compensated for the time differences by reducing the number of leap years by mandating the centurial years (for example, 1800, 1900, 2000, etc.) would not be leap years unless divisible by 400. So, for example, the year 1900 was not a leap year, while the last centurial year, 2000, was. The final decision was to implement the calendar correction in one move (as proposed by Clavius), and this was the advice implemented by Pope Gregory.
 
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<img height='375' src='https://upload.wikimedia.org/wikipedia/commons/8/85/Biccherna_Tablet_-_Commission_for_the_Reform_of_the_Calendar_%28Pope_Gregory_XIII_Presiding%29.jpg'> 
Commission for the Reform of the Calendar (Pope Gregory XIII Presiding) 
Artist Unknown 
From Wikimedia Commons, in the Public Domain</center>
 
An additional change that was implemented was to readjust the calendar so that in 1582, Easter fell on March 21 (as it had when the First Council of Nicaea fixed the vernal equinox). As a result, 10 days were “removed” from the calendar to bring it back into line with the seasons. This change occurred in October, when Thursday, October 4, 1582, was followed by Friday, October 15. Initially, only four countries adopted the new calendar in 1582, although others soon followed (as a note, England did not make the change until 1752).
 
There is one more interesting historical note tied to the Church and one of its saints due to the change to the Gregorian Calendar. The Spanish Carmelite nun and church reformer Saint Teresa of Avila’s date of death is sometimes listed as October 4, 1582, or as October 15, 1582. She apparently died either right before midnight on October 4, or right after midnight, moving her date of death forward by the ten removed days to October 15.
 
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<img height='275' src='http://vaticanstamps.org/VPS/images/296/1494.jpg' >
<img height='275' src='http://vaticanstamps.org/VPS/images/296/711.jpg' > 
German mathematician Father Christopher Clavius (Scott 1494, 2012) 
Saint Teresa of Avila, 400th Anniversary of Death (Scott 711, 1982)</center>
 
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REFERENCES: 
<li>Anonymous, Vatican Notes, Volume 31, Number 4, pages 1 and 8, 1983, <a href='https://vaticanstamps.org/vaticannotes/idisplay2.php?p=186-01&r=0'>Calendar Reform</a> 
<li>Look and Learn, <a href='https://www.lookandlearn.com/blog/3370/pope-gregory-xiii-creates-the-gregorian-calendar/'>Pope Gregory XIII Creates the Gregorian Calendar </a> 
<li>Wikipedia, <a href='https://en.wikipedia.org/wiki/Gregorian_calendar'> Gregorian Calendar</a> 
<li>James C. Hamilton, Vatican Notes, Volume 67, Number 380, pp. 36-43, 2019, <a href='http://vaticanstamps.org/vaticannotes/idisplay.php?p=67-380-36-37&r=0'>400th Anniversary Of The Gregorian Calendar, 1982 A First Day Cover Survey</a> 
<li>Vatican Philatelic Society website, www.vaticanstamps.org, Stamp Database Search

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