Margen
Imagine going to sleep on a Thursday and having the calendar say tomorrow is the 15th. Ten days gone in one stroke. That's what happened to half of Europe in 1582.
The Julian calendar, inherited from Rome, reckoned the year just a touch too long. Every century piled up a small drift that, after more than a thousand years, added up to ten extra days compared with the seasons. Easter kept sliding, and that worried the Church.
To fix it, Pope Gregory XIII ordered a leap: Thursday 4 October 1582 was followed straight away by Friday the 15th. In Spain, Portugal and Italy, those ten days simply didn't exist. No one was born, no one died, no one worked during them. And the calendar that crossed off that batch of days is, with tiny tweaks, the very same one you use today.