The pope reforms the calendar
On this day · 24 February 1582
On this day in 1582, a papal bull fixed the slipping seasons and gave us the calendar most of the world still keeps.
On 24 February 1582, Pope Gregory XIII issued the bull Inter gravissimas, reforming the centuries-old Julian calendar. The problem was arithmetic: the Julian year ran slightly too long, and over twelve centuries the spring equinox had drifted about ten days out of place, dragging the date of Easter with it.
The fix was blunt. To snap the seasons back into line, ten days were simply deleted from that October — Thursday, 4 October was followed by Friday, 15 October 1582.
To stop the drift recurring, the reform also dropped most century leap years, keeping only those divisible by 400.
Catholic states adopted it quickly; Protestant and Orthodox lands resented a Roman decree and dragged their feet for generations — Britain and its colonies held out until 1752. Today this Gregorian calendar is the civil standard across nearly the entire world.
Sources & references
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