Pope Gregory XIII introduced the Gregorian Reform of the Calendar on February 24, 1582. The Bull containing this reform has been nicknamed, Inter Gravissimas, after the first two words of the text. These words mean, "Among the most serious . . . " The first sentence reads, Among the most serious tasks, last perhaps but not least of those which in our pastoral duty we must attend to, is to complete with the help of God what the Council of Trent has reserved to the Apostolic see.
		Pope Gregory 13 (Ugo Boncampagni)

With these words, Gregory expressed his vision of the 1582 calendar reform as completing a task called for by the Council of Trent in 1563.

Here is the first page of Inter Gavissimas, as reproduced in Christoph Clavius's Opera Mathematicorum: Inter Gravissimas (jpg 230kb)

Here is a transcription of the Latin text of Inter Gravissimas; and an English translation.