The Gregorian calendar took effect
On this day · 15 October 1582
To drag the calendar back into step with the seasons, the pope simply deleted ten days — and millions went to sleep in one date and woke in another.
In Catholic Europe, Thursday, October 4, 1582, was followed directly by Friday, October 15, 1582. Ten days vanished overnight, by decree of Pope Gregory XIII, marking the debut of the Gregorian calendar still used worldwide today.
The old Julian calendar overestimated the length of the year by about 11 minutes. Over centuries that tiny error compounded, pushing the spring equinox roughly ten days off its proper date and slowly unmooring Easter from its intended season. Dropping ten days snapped the equinox back to March 21.
October was chosen deliberately — it held no major feast days to disrupt.
Spain, Portugal, Poland, and the Italian states switched on schedule; France waited until December. Protestant and Orthodox nations, wary of a papal reform, resisted for centuries — Britain didn’t convert until 1752, by which point it had to skip eleven days.
Sources & references
2 referencesWell-established. Corroborated by 2 independent sources.
