Pope Gregory XIII issued his papal bull establishing the use of the Gregorian Calendar throughout Western Christendom
Actually, this is not entirely correct. The Gregorian Calendar was not adopted throughout Western Christendom for two centuries. In fact, not even the majority
of countries in Western Christendom adopted it- only Spain, Portugal, the Polish-Lithuanian Commonwealth and parts of Italy adopted it in 1582. England, Ireland, and the British colonies kept the Julian Calendar until 1752 (two centuries after the Bull).
The reason for this was that the Bull was seen as only having authority in the Papal States.