Inter gravissimas

Gregory XIII issues Inter gravissimas ordering calendar reform

The document that established the calendar we call Gregorian is itself dated 1581, because its year began on 25 March.

Dated at Tusculum, 24 February 1582

Pope Gregory XIII dated the bull Inter gravissimas to 24 February 1582 in modern year-numbering, though its closing clause reads 1581 because it used an Annunciation-style year beginning on 25 March.

That date is Julian, equivalent to 6 March in the proleptic Gregorian calendar — a retrospective conversion, since the new calendar was not yet in legal use. A document held by the Italian Chamber of Deputies records that Gregory signed on 24 February but had the bull publicly affixed and promulgated on 5 March, so 24 February is safest described as its signing date.

Ten days deleted, three century leap days in four

The bull ordered ten dates omitted — 5 through 14 October 1582 — so that 4 October would be followed by 15 October while the weekday sequence continued normally. It kept leap years every four years but directed that three of every four century years cease to be leap years, with 1600 and every fourth century year after it retained.

It also replaced the old lunar 'golden number' mechanism with a cycle of epacts, meant to restore the ecclesiastical full moon used to calculate Easter. The bull identified about ten days of drift since the Council of Nicaea and a lunar displacement of four days or more.

The clause about debts

Gregory did not simply delete dates. The bull ordered judges to add ten days to affected payment deadlines so that the jump would not cause financial harm to those owing money.

No civil dates changed that February. The bull authorized printing and dissemination of the corrected calendar and directed Catholic institutions to make the October change, allowing later implementation where notice arrived too late.

Within papal jurisdiction, unauthorized printers faced loss of the books and a penalty of 100 gold ducats.

Sources

Researched 24 Aug 2026 5 sources not yet audited Traditional date

How this was checked

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Traditional date. Conventionally dated here; the research places it at 1582-02-24.

What the sources leave uncertain

The traditional issuance date is 24 February 1582 Julian, equivalent to 6 March in the proleptic Gregorian calendar. The new calendar was not yet legally in use, so “6 March Gregorian” is a retrospective conversion.
The bull's closing clause says 1581 because it used an Annunciation-style year beginning on 25 March; modern year-numbering renders the date 1582.
A source in the Italian Chamber of Deputies records that Gregory signed the bull on 24 February but had it publicly affixed and promulgated on 5 March. Accordingly, 24 February is safest described as the bull's documentary or signing date, not necessarily the day the public first saw it.