“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. ...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 [II], 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.
...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 anyone shall have presumed to attempt this, he shall have acquainted the incursion the wrath of Almighty God, as even His blessed apostles Peter and Paul.” Pope Gregory XIII, Inter Gravissimas, February 24, 1581