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Inter gravissimas is a papal bull issued by Pope Gregory XIII on February 24, 1582. It is the founding document for the Gregorian calendar, which is used in most countries.

This bull declares that:

  • October 4, 1582 will be followed by October 15, 1582 (paragraph 7);
  • these 10 missing days will not be counted in calculating end days of loans, etc. (paragraph 8);
  • centennial years, such as 1700, 1800, and 1900 will not be leap years, but years that can be divided by 400, such as 1600 and 2000 will be (paragraph 9);
  • Easter will be computed via new Paschal tables (paragraph 10).

The name of the bull consists of the first two words of the bull, which starts: "Inter gravissimas pastoralis officii nostri curas " ("Among our most serious pastoral duties ").

Attached to the bull were six canons which enabled the computation of the new Gregorian Easter and two calendars listing saints' days, one for the last 2½ months of 1582 and another for the entire new Gregorian year. The bull, canons, and calendars were reprinted as part of the front matter of the principal book explaining and defending the Gregorian calendar, Christoph Clavius, Romani calendarij à Gregorio XIII. P. M. restituti explicatio (1603), which is tome V in his collected works Opera Mathematica (1612), available in the microfiche collection Landmarks II monographs.

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