Pope Gregory Reforms the Calendar Introduced by Julius Caesar in 45BCE
October 15, 1582
On 15 October 1582, Pope Gregory XIII enacts a major reform of the Julian calendar, introduced by Julius Caesar in 45 BCE.
To correct the calendar’s drift with respect to the solar year, 10 days are removed, making 4 October followed directly by 15 October 1582. This new system, the Gregorian calendar, aligns more accurately with the Earth’s revolutions around the Sun and adjusts leap years accordingly.
The reform is immediately adopted by Catholic countries such as Spain, Italy, and Portugal. However, Britain and Ireland, being Protestant, do not adopt the change until 1752, by which time the calendar was out of sync by 11 days.
This change also led to public confusion and unrest in some places, with reports of people demanding: “Give us back our eleven days!”