Inter gravissimas was a "papal bull issued by "Pope Gregory XIII on February 24, 1582. The document, written in "Latin, reformed the "Julian calendar. The reform came to be regarded as a new calendar in its own right and came to be called the "Gregorian calendar, which is "used in most countries today.
The intention expressed by the text of this bull was "to restore" the calendar so that seasonal events critical for the calculation of Easter dates would be back in their "proper places" and would be prevented from being moved away again. The idea of reform as such is not otherwise mentioned. The bull identifies "three necessary" things for the correct determination of Easter dates: correct placement of the northern vernal equinox; correct identification of the "14th day of the moon" (effectively full moon) that happens on or next after the vernal equinox, and the first Sunday that follows that full moon. The first two items were the ones that received attention; the third, about choosing the next following Sunday, was not identified as causing any problem, and was not further mentioned.
By "restore", Gregory meant two things. First, he adjusted the calendar so that the "vernal equinox was near March 21, where it had been during the "Council of Nicaea (May 20 – August 25, 325). This required removing ten days of drift. Second, he made the tabular 14th day of the moon correspond with the real full moon, removing "four days and more" of drift. This would restore the dates of Easter to near where they were at the time of the Council of Nicaea, although that council had not specified where in the calendar the vernal equinox should fall and had not adopted any particular type of lunar tables. The practices of the "Roman Catholic Church that had become traditional by 1582 for calculating the Easter and lunar calendars became settled when "Dionysius Exiguus translated the rules of the "Church of Alexandria from "Greek into "Latin in 525. ("Northumbria adopted them at "Whitby in 664, the "Welsh around 768, and France around 775. Before this, France and Rome had used "Victorius's less exact 457 translation of the Alexandrian calendar; Britain and Rome before Victorius had used "Augustalis's 84-year cycle.)
Gregory also made changes to the calendar rules, intending to ensure that, in the future, the equinox and the 14th day of the Paschal moon, and consequently Easter Sunday, would not move away again from what the bull called their proper places.
The changes (relative to the Julian calendar) were as follows:
The name of the bull consists of the first two words of the bull, which starts: "Inter gravissimas pastoralis officii nostri curas…" ("Among the most serious duties of our "pastoral office…").
The bull refers to "the explanation of our calendar" and to a canon related to the dominical letter. To accompany the bull there were six chapters of explanatory rules ('canons'), and some of these (canons 1, 2, 4) refer to a book entitled Liber novæ rationis restituendi calendarii Romani  for a fuller explanation of the tables than that contained in the canons (or the bull). Because the bull and canons refer to each other, they must have been written at roughly the same time, printed at the same time (March 1), and distributed to the several countries together.
These canons enabled the computation of Easter dates in the reformed ('restored') Gregorian calendar, and gave two calendar-listings "saints' days, one for the 'year of correction' (1582) and another for the entire new Gregorian year. The bull, canons, and calendars were reprinted as part of the principal book explaining and defending the Gregorian calendar, "Christoph Clavius, Romani calendarii a Gregorio XIII. P. M. restituti explicatio (1603), which is tome V in his collected works Opera Mathematica (1612).
The version of "Inter gravissimas" included by Christoph Clavius in his work explaining the Gregorian calendar contained these dating clauses: "Anno Incarnationis Dominicae M. D. LXXXI. Sexto Calend. Martij, Pontificatus nostri Anno Decimo. ... Anno à Natiuitate Domini nostri Iesu Christi Millesimo Quingentesimo Octuagesimo secundo Indictione decima,". These clauses include four years:
All of these years agree that the bull was dated February 24, 1582, using the modern January 1 beginning of the year.
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Gregory's reform was enacted in the most solemn of forms available to the Church, but the bull had no authority beyond the "Catholic Church and the "Papal States. The changes which Gregory was proposing included changes to the "civil calendar over which Gregory had no authority (except in the Papal States). The text of the bull recognized this by giving what amounted to orders to the clergy and those "presiding over churches": but in contrast, where the text addresses the civil authorities ("kings, princes and republics"), it "asks", "exhorts" and "recommends" the new calendar changes. The changes required adoption by the civil authorities in each country to have legal effect.
The bull Inter gravissimas was immediately adopted by the major Catholic powers of Europe, but the Protestant countries refused to adopt it until the 18th century, and Eastern European countries adopted it only during or after World War I (the last European country to adopt it was Greece, in 1923).
Most "Orthodox Churches have not adopted it at all and continue to reckon their ecclesiastical years by the Julian calendar, even though their home countries use the Gregorian calendar for civil purposes.