Inter gravissimas in the context of "Gregorian Calendar"

The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years slightly differently to make the average calendar year 365.2425 days long rather than the Julian calendar's 365.25 days, thus more closely approximating the 365.2422-day "tropical" or "solar" year that is determined by the Earth's revolution around the Sun.

The rule for leap years is that every year divisible by four is a leap year, except for years that are divisible by 100, except in turn for years also divisible by 400. For example, 1800 and 1900 were not leap years, but 1600 and 2000 were leap years.

The adoption of the Gregorian Calendar has taken place in the history of most cultures and societies around the world, marking a change from one of various traditional (or "old style") dating systems to the contemporary (or "new style") system – the Gregorian calendar – which is widely used around the world today. Some states adopted the new calendar in 1582, others not before the early twentieth century, and others at various dates between. A few have yet to do so, but except for these, the Gregorian calendar is now the world's universal civil calendar, old style calendars remaining in use in religious or traditional contexts. During – and for some time after – the transition between systems, it has been common to use the terms "Old Style" and "New Style" when giving dates, to indicate which calendar was used to reckon them.

The Gregorian calendar was decreed in 1582 by the papal bull Inter gravissimas by Pope Gregory XIII, to correct an error in the Julian calendar that was causing an erroneous calculation of the date of Easter. The Julian calendar had been based upon a year lasting 365.25 days, but this was slightly too long; in reality, it is about 365.2422 days, and so over the centuries, the calendar had drifted increasingly out of alignment with the Earth's orbit. According to Gregory's scientific advisers, the calendar had acquired ten excess leap days since the First Council of Nicaea (which established the rule for dating Easter in AD 325). Consequently, he ruled, the numbering of days must jump by ten, to restore the status quo ante; thus, for example, when the Catholic countries of Europe adopted the new calendar, the day after Thursday, 4 October 1582 was Friday, 15 October 1582. Countries which did not change until the 18th century had by then observed an additional leap year (1700), necessitating the removal of eleven days from the reckoning. Some countries did not change until the 19th or 20th century, necessitating the removal of one or two further days.

1582 (MDLXXXII) was a common year starting on Monday in the Julian calendar, and a common year starting on Friday (link will display full calendar) of the Proleptic Gregorian calendar. This year saw the beginning of the Gregorian calendar switch, when the papal bull Inter gravissimas introduced the Gregorian calendar, adopted by Spain, Portugal, the Polish–Lithuanian Commonwealth and most of present-day Italy from the start. In these countries, the year continued as normal through Thursday, October 4; the next day became Friday, October 15, like a common year starting on Friday. France followed two months later, letting Sunday, December 9 be followed by Monday, December 20. Other countries continued using the Julian calendar, switching calendars in later years, and the complete conversion to the Gregorian calendar was not entirely done until 1923.