In October 1582, ten days simply disappeared from the calendar.
To fix the drift of the old Julian calendar, Pope Gregory XIII decreed that Thursday 4 October 1582 would be followed directly by Friday 15 October. Nobody in Catholic Europe was born or died on the dates in between.
Why is this true?
The Julian year was about 11 minutes too long, and after 1,600 years the calendar had slipped ten days against the seasons. Protestant countries resisted the change for a long time; Britain did not switch until 1752, losing eleven days.
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More about history
History keeps producing facts that ruin your sense of time. A war shorter than a lunch break, an empire that overlaps with the invention of the fax machine, a queen who lived closer to the Moon landing than to the pyramids. The entries here favour events with solid documentation, and each one explains the context that makes the surprise make sense rather than leaving it as a bare curiosity.