The Gregorian calendar began, skipping ten days
On this day · 4 October 1582
To fix a slowly drifting year, Pope Gregory XIII deleted ten dates outright, so October 4 was followed by October 15.
In Catholic countries, Thursday, October 4, 1582, was followed the next morning not by the 5th but by Friday, October 15. Ten dates, the 5th through the 14th, simply never happened.
The deletion fixed a long-standing error. The Julian calendar overestimated the solar year by about 11 minutes, and over centuries that surplus had pushed the spring equinox roughly ten days off its assigned date of March 21, scrambling the calculation of Easter. Pope Gregory XIII, by the bull Inter gravissimas (issued February 24, 1582), ordered the days dropped to snap the equinox back into place.
Ten days vanished by decree, and almost nobody noticed the difference the morning after.
The reform also tightened the leap-year rule: century years count as leap years only when divisible by 400. That refinement keeps the calendar accurate to about one day every 3,300 years, which is why we still use it.
Sources & references
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