Папская булла Inter gravissimas и введение григорианского календаря

Papal document that established the Gregorian calendar

Inter gravissimas (English: "Among the most serious ") was a papal bull issued by Pope Gregory XIII on 24 February 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 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:
"Anno Incarnationis Dominicae M. D. LXXXI." (In the year of the Incarnation of the Lord 1581) is the year beginning 25 March 1581. 25 March is the traditional date of the conception, annunciation, and incarnation of Jesus. "Sexto Calend. Martij" (On the sixth [day] before the Kalends [1st] of March.) The Romans counted inclusively; modern usage would say the fifth day before 1 March, or 24 February. "Pontificatus nostri Anno Decimo" (In the tenth year of our pontificate) is the year beginning 13 May 1581. Gregory XIII was elected pope on 13 May 1572 (in terms of the Julian calendar). "Anno à Natiuitate Domini nostri Iesu Christi Millesimo Quingentesimo Octuagesimo secundo" (In the year from the Nativity of our Lord Jesus Christ 1582) is the year beginning 25 December 1581 using the modern 1 January beginning of the year. It marks the beginning of year 1582 if that year began on 25 December, the traditional date of the birth or nativity of Jesus. "Indictione decima" (Indiction 10) is the year beginning 1 January, which agrees with modern reckoning. All of these years agree that the bull was dated 24 February 1582, using the modern 1 January beginning of the year.

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 Eastern Orthodox Churches and Oriental 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.