A calendar introduced by Pope Gregory XIII in the year 1582. The calendar is based upon the Julian calendar. Ten days from the Julian calendar were surpressed by the switch to the Gregorian calendar. The years consist of 365 days with 366 in Leap Years which occur in years that are devisible by four with the exception that years that are multiples of 100 are only Leap Years if they are also divisible by 400. The Gregorian calendar is the calendar that is still in common use today.

We can calculate the "average year length" of a Gregorian calendar year. The calendar cycles every 400 years, so this is just the average length of a year over a single cycle.

How many leap years do we get in one cycle? 100, except for 4, except for 1, or 97 leap years. And the length of an ordinary year is 365 days, while a leap year is 366 days. Averaging out, we get

```97 * 366  +  (400-97) * 365
---------------------------  =  365.2425
400
```
The correct (official; there are several ways of measuring this value, depending exactly what you'd like to get) value for the tropical year is currently around 365.24219, so the calendar is 0.00031 days too fast (each year). In other words, it gains almost a third of a day every 1000 years.

But of course, the value of the tropical year also changes...

Friday, October 15, 1582 was the first day of the modern Gregorian calendar. The previous day was Thursday, October 4, 1582 of the Julian calendar.

While very similar to the Gregorian calendar, the Julian calendar contained 365 days a year, and added a leap day in February every four years. Unfortunately, a solar year is actually 365.2422 (mean tropical year) to 365.2424 (vernal equinox year) days long. With the Julian calendar assuming it was 365.25 days long, every 128 to 131 years, the calendar would gain an extra day. After several centuries of use, it became noticeable that the seasons were getting out of sync with the calendar. The Roman Catholic Church was especially troubled that Easter was occurring at the wrong time of year.

To solve this problem, Pope Paul III enlisted the aid of several astronomers including Christopher Clavius to try and solve this problem. Built upon the ideas of the deceased physician and astronomer Luigi Lilio and later enacted under the next leader of the church, Pope Gregory XIII, the new Gregorian system made the following changes:

• Ten days were removed from the calendar, and it was decreed that the day following Thursday, October 4, 1582 (i.e., October 5, 1582 of the Julian calendar) would be known as Friday, October 15, 1582.
• Instead of a simply determining a year was a leap year if it was divisible by four, in the Gregorian calendar a year is a leap year if either:
• it is divisible by 4 but not by 100
or
• it is divisible by 400.

Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not.

• New rules for determining the date of Easter were adopted.
• The position of the extra day in a leap year was moved from the day before February 25th to the day following February 28th.

The Catholic countries of Italy, Spain, Portugal, Poland, France, and Luxembourg quickly made the switch to the new calendar, other Catholic countries followed within the next several years. Since this change was created by the Roman Catholic Church, numerous Protestant and Greek Orthodox countries did not make the switch for several decades or even centuries. This causes some serious confusion for historians and genealogists including simple things like calculating the day of the week for a certain date. Below is a chart listing the dates that several nations officially adopted the new calendar system.

```Country                    End Julian Calendar   Begin Gregorian Calendar
Albania                    Dec     1912          Dec     1912
Austria
Tyrol                  Oct  5, 1583          Oct 16, 1583
Carinthia, Styria      Dec 14, 1583          Dec 25, 1583
Belgium
Spanish Provinces      Dec 21, 1582          Jan  1, 1583
Liège                  Feb 10, 1583          Feb 21, 1583
Bohemia (Czech Republic)   Jan  6, 1584          Jan 17, 1584
Bulgaria                   Nov  1, 1915          Nov 14, 1915
China                      Dec 18, 1911          Jan  1, 1912
Canada                     Sep  2, 1752          Sep 14, 1752
Denmark                    Feb 18, 1700          Mar  1, 1700
Færø Islands           Nov 16, 1700          Nov 28, 1700
Egypt                              1875                  1875
Estonia                    Feb  1, 1819          Feb 15, 1819
Finland                    Feb 17, 1753          Mar  1, 1753
France                     Dec  9, 1582          Dec 20, 1582
Alsace                         1648                  1648
Strasbourg             Feb  5, 1682          Feb 16, 1682
Germany, Catholic Regions
Augsburg               Feb 13, 1583          Feb 24, 1583
Baden                  Nov 16, 1583          Nov 27, 1583
Bavaria                Oct  5, 1583          Nov 16, 1583
Cologne                Nov  3, 1583          Nov 14, 1583
Jülich                 Nov  2, 1583          Nov 13, 1583
Mainz                  Nov 11, 1583          Nov 22, 1583
Münster, Strasbourg    Nov 16, 1583          Nov 27, 1583
Trier                  Oct  4, 1583          Oct 15, 1583
Würzburg               Nov  4, 1583          Nov 15, 1583
Germany, Protestant Regions
Hildesheim             Mar 15, 1631          Mar 26, 1631
Kurland                        1617                  1617
Minden                 Feb  1, 1668          Feb 12, 1668
Neuburg                Dec 13, 1615          Dec 24, 1615
Osnabrück                      1624                  1624
Paderborn              Jun 16, 1585          Jul 27, 1585
Prussia                Aug 22, 1610          Sep  2, 1610
Westphalia             Jul  1, 1584          Jul 12, 1584
All Others             Feb 18, 1700          Mar  1, 1700
Great Britain              Sep  2, 1752          Sep 14, 1752
& American colonies
Greece                     Sep 14, 1916          Sep 28, 1916
Holy Roman Empire          Jan  6, 1584          Jan 17, 1584
Hungary                    Oct 21, 1587          Nov  1, 1587
Transylvania           Dec 14, 1590          Dec 25, 1590
Iceland                    Nov 16, 1700          Nov 28, 1700
Italy                      Oct  4, 1582          Oct 15, 1582
Japan                              1873                  1873
Latvia                     Feb  1, 1918          Feb 15, 1819
Lithuania                  Feb  1, 1918          Feb 15, 1819
Moravia (Czech Republic)   Jan  6, 1584          Jan 17, 1584
The Netherlands
Holland, North Brabant Dec 21, 1582          Jan  1, 1583
Gelderland, Zutphen    Jun 30, 1700          Jul  7, 1700
Utrecht, Overijssel    Nov 30, 1700          Dec 12, 1700
Friesland, Groningen   Dec 31, 1700          Jan 12, 1701
Drente                 Apr 30, 1701          May 12, 1701
Norway                     Feb 18, 1700          Mar  1, 1700
Poland                     Oct  4, 1582          Oct 15, 1582
Silesia                Jan 12, 1584          Jan 23, 1584
Portugal                   Oct  4, 1582          Oct 15, 1582
Romania                    Mar 31, 1919          Apr 14, 1919
Transylvania            Dec 14, 1590          Dec 25, 1590
Russia                     Jan 31, 1918          Feb 14, 1918
Spain                      Oct  4, 1582          Oct 15, 1582
American Colonies              1584                  1584
Sweden                     Feb 17, 1753          Mar  1, 1753
Switzerland
Lucern, Uri, Schwyz,   Jan 11, 1584          Jan 22, 1584
Zug, Freiburg,
Solothurn
Wallis                 Feb 28, 1655          Mar 11, 1655
Zürich, Bern, Basel,   Dec 31, 1700          Jan 12, 1701
Schaffhouse, Geneva,
Thurgovia
Appenzell, Glarus,             1724                  1724
St. Gallen
Turkey                             1927                  1927
United States
British Colonies       Sep  2, 1752          Sep 14, 1752
Alaska                 Oct  5, 1867          Oct 18, 1867
Yugoslavia                 Mar  4, 1919          Mar 18, 1919
```

References

• http://www.searchforancestors.com/utility/gregorian.html
• http://serendipity.magnet.ch/hermetic/cal_stud/cal_art.htm

When eleven days were omitted from the year 1752 in the introduction of the Gregorian Calendar to England, Wednesday September 2 was followed by Thursday September 14, not, as it otherwise would have been, Monday the 14th.

Simply omitting the dates caused riots in the streets. It was therefore probably a wise move not to interfere with the week, which is a religious thing

There are millions of devout people of various religious persuasions - certainly Jewish and Christian - for whom there has been a Sabbath every seven days since the Creation. You don't monkey with that if you value your life.

This dislocation meant that 1752 became the only year in history to have a triple dominical letter - EDA. This records the fact that the alignment between dates and days of the week "slipped" once at the end of February (as is normal for a leap year - all leap years have a double letter) and again with the missing days in September

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