Inter Gravissimas
Issued by Pope Gregory XIII, February 24, 1581/2
Reproduced in Christoph Clavius’s Opera Mathematicorum

Gregorius episcopus servus servorum Dei, ad perpetuam rei memoriam. Inter gravissimas pastoralis officii nostri curas, ea postrema non est, ut quæ a sacro Tridentino concilio Sedi Apostolicæ reservata sunt, illa ad finem optatum, Deo adiutore, perducantur.

Gregory, bishop, servant of the servants of God, in perpetual memory. Among our serious pastoral duties, not the last is that we care to complete those sacred rites reserved by the Council of Trent [1545-63], with the guiding assistance of God. As the council fathers also applied their attention and ultimate reflections to the breviary, but were stopped by the lack of time, they wisely decided to refer this entire question to the authority and the judgment of the Roman pontiff.

However there are two principal parts in the breviary: the one comprises the prayers and the religious anthems to recite on the public holidays and the working days; the other concerns the annual cycle of Easter and other festivals whose return depends on measuring the movement of the sun and the moon. The reform of the first part, Pius V [1566-1572], our predecessor of happy memory, discharged and put it into force. The reform of the second part, which first requires the restoration of the calendar, was tried often, over a long time, by our predecessor Roman pontiffs.  However, it could not be carried out, thus far, because the various calendar reform projects proposed by astronomers, in addition to presenting the immense and almost inextricable difficulties which always accompanied such a reform, were not durable, and especially they did not maintain intact the ancient rites of the Church, and that was our first concern in this business.

While, therefore, this has been entrusted to us, an undeserved dispensation from God has been allowed.  Our dear son Antonio Lilio, professor of science and medicine, brought to us a book, written at one time by his brother Aloysius [Luigi], in which this one showed that, by means of a new cycle of epacts which he had devised, and who directed his own particular Golden Number pattern and accomodated the entirety of any solar year, every [defect of] the calendar collapsed, and the constant calculations would endure for every generation. He was, thus, able to restore and explain how the calendar itself will never need published any further change. This new project of the restoration of the calendar, summarized in a small book, we forwarded a few years ago to the Christian princes and to the large universities so that this work, which is the business of all, is carried out with the consultation of all.  Those having expressed their agreement to us, as we had sincerely hoped, we have, with this consensus, arranged a gathering in the Holy City, to reform the calendar, of the very qualified men on the matter whom we had chosen from the principal countries of the Christian world a long time before.  Those, after having devoted much time and attention to this night work and having discussed between them cycles which they had collected from everywhere, old ones as well as modern ones, and as they had carefully studied the reflections and the opinions of erudite men who wrote on this subject, chose and prefered this cycle of epacts, adding to it elements which, after thorough examination, appeared essential to the realization of a perfect calendar.

One notes in examining this that it is necessary to rule at the same time on three points to restore the celebration of Easter according to rules fixed by the previous Roman pontiffs, particularly Pius I and Victor I, and by the fathers of the councils, in particular those of the great ecumenical council of Nicæa [AD 325]. Namely: First, the precise date of the vernal equinox, then the exact date of the fourteenth day of the moon which reaches this age the very same day as the equinox or immediately afterwards, finally the first Sunday which follows this same fourteenth day of the moon. Therefore we took care not only that the vernal equinox returns on its former date, of which it has already deviated approximately ten days since the Nicene Council, and so that the fourteenth day of the Paschal moon is given its rightful place, from which it is now distant four days and more, but also that there is founded a methodical and rational system which ensures, in the future, that the equinox and the fourteenth day of the moon do not move from their appropriate positions.

So thus that the vernal equinox, which was fixed by the fathers of the Nicene Council at XII calends April [March 21], is replaced on this date, we prescribe and order that there is removed, from October of the year the 1582, the ten days which go from the third before Nones [the 5th] through the day before the Ides [the 14th] inclusively. The day which will follow IV Nones [the 4th], when one traditionally celebrates Saint Francis, shall be the Ides of October [the 15th], and there shall be celebrated this day the festival of the martyred Saints Denis, Rusticus, and Eleutherius, as well as the memory of Saint Mark, pope and confessor, and of the martyred Saints Sergius, Bacchus, Marcellus and Apuleius. There shall be celebrated the following day, seventeenth calends November [October 16th], the festival of Saint Callistus, pope and martyr [feast day October 14]; and then shall come XVI calends November [October 17th], the office and the mass of the 18th Sunday after Pentecost; the Sunday letter shall pass from G to C. Finally shall come the place of the fifteenth calends November [October 18th], the festival of Saint Luke the evangelist [feast day October 18], after which will follow one another the other feastdays, in the way they are described in the calendar.

But so that this ten days removal does not cause any injury with whomever must carry out monthly or annual payments, it will fall to the judges, in any litigation which could result from it, to take account of the aforementioned removal by deferring ten days the expiration of any payment.

Then, lest the equinox recede from XII calends April [March 21st] in the future, we establish every fourth year to be bissextile (as the custom is), except in centennial years which always were bissextile until now; we wish that year 1600 is still bissextile; after that, however, those centennial years that follow are not all bissextile, but in each four hundred years, the first three centennial years are not bissextile, and the fourth centennial year, however, is bissextile, so the years 1700, 1800 and 1900 will not be bissextile. Assuredly, the year 2000, as with our custom, will have a bissextile intercalation, February will contain 29 days, and the same rule of intermittent bissextile intercalations in each four hundred year period will be preserved in perpetuity.

Moreover, so that the fourteenth day of the Paschal moon is given with precision and that the age of the moon is presented with precision to the faithful in accordance with the antique use of the Church, to take note of it each day with the reading of martyrology , we order that once the Golden Number is withdrawn from the calendar, one substitutes the cycle of the epacts for it which, thanks to its very precise rules mentioned above for the Golden Number, makes so that the new moon and the fourteenth day of the Paschal moon always hold their place. And this is seen clearly in the explanation of our calendar, where are also presented Paschal tables in conformity with the ancient habits of the Church and which make it possible to find more surely and more easily the sacred date of the Easter.

Lastly, partly because of the ten days cut off from October from year 1582 (that one must now call the year of the reform) and partly because of the three days which will not have to be inserted any more during each four hundred year period, it will be necessary to stop the 28 year cycle of the Sunday letters used so far in the Roman Church.  We wish to substitute for it the same 28 year cycle, as adapted by this same Lilio to the rule of bissextile intercalation for the last years of the century as well as to the whole solar year, so that the Sunday letter can always be given as easily as before using the solar cycle, as that is explained in the canon which is referred to it.

We, therefore, with what is traditionally the attribute of sovereign pontiff, we approve this calendar, now reformed and made perfect thanks to the infinite benevolence of God towards his Church, and we order that it is printed in Rome with the martyrology.

But so that these orders are maintained intact and clear everywhere of faults and errors, we prohibit all the printers established on the territory subject, with or without intermediary, to our jurisdiction and that of holy Roman Church, to have the audacity or the presumption to print or publish, without our authorization, the calendar or martyrology, together or separately, or to draw benefit in any manner from it, under penalty of the loss of contracts and of a fine of one hundred gold ducats to pay to the Apostolic See.  As for others, where that they remain similarly subject, we give them the same prohibition, under penalty of excommunication latæ sententiæ and under other sorrows within our discretion.

We thus remove and absolutely abolish the old calendar and we wish that all the patriarchs, primacies, archbishops, bishops, abbots and other leaders of Churches put into force for the reading of the divine office and the celebration of the festivals, each one in his Church, monastery, convent, command, army or diocese, the new calendar, to which was adapted the martyrology, and make use only of this one, as well as all the other priests and clerks, secular and regular, of the both genders, as well as soldiers and all Christians, this calendar whose use will start after the ten days removal of October 1582. As for those however which live in areas too distant to take knowledge of this letter in time, they are allowed to make such a change in October of the year which will follow immediately, namely 1583, or the next one, as soon, of course, as this letter will have come to them, in the manner that we indicated above and as that will be more abundantly explained in the calendar of the year of the reform.

In addition, under the terms of the authority with which we were invested by God, we exhort and request our very dear sons in Christ, our illustrious Rudolph, King of the Romans become emperor, as well as the other kings and princes, just as the republics. We recommend to them, both since they sought us to achieve this very admirable work, but also, and even especially, in order to maintain harmony between the Christian nations in the celebration of the festivals, to adopt for themselves our calendar and to take care that all their subjects adopt it respectfully and conform to it scrupulously.

As it would be difficult however to forward this letter to all the countries of the Christian world, we order that it is made public and displayed on the gates of the basilica of the Apostolic Prince and on those of the Apostolic Chancellery, as well as at the entry of Campo di Fiori. Also, among all people and in all the countries, one shall grant the same absolute credit to copies of this letter, even printed, accompanied by specimens by the calendar and martyrology mentioned previously, at the same time with the signed hand and notary seal of a public and authenticated dignitary of the Church, that that which would be granted by all to the displayed original letter.

No one among men, therefore, is permitted to infringe on this page, our prescription, mandate, establishment, wish, approval, prohibition, suppression, abolition, exhortation and request, nor dare reckless opposition. But if you tamper with this, Almighty God will take you up with indignation, and you will be making an incursion against His blessed apostles Peter and Paul.

Given at Tusculum, in the year of the Incarnation of the Lord 1581, sixth calends March [February 24th], of our pontificate, year 10.

Cae. Glorierius  
A. de Alexijs  

Datum Tusculi, anno Incarnationis dominicæ MDLXXXI, sexto Kalendas Martii, pontificatus nostri anno X.

[The date is in the Easter year style of Florence which started each year on 25th March, so in the modern calendar this would be 1582.]

Translation from Latin courtesy of Bill Spencer, November 24-28, AD 1999 / March 20, AD 2002

Accessed January 2012 at>