Ten days vanished when Europe switched calendars in 1582
Pope Gregory XIII did not invent a new calendar from scratch. He trimmed the Julian drift with a clever leap-year rule and deleted ten days overnight so Easter would stay tied to spring across Catholic Europe and beyond.
The Gregorian calendar, introduced by papal bull Inter gravissimas in October 1582, replaced the Julian system used across much of the world. Its key fix was leap-year spacing: years divisible by four leap, yet century years leap only when divisible by 400. Thus 1600 and 2000 leap; 1800 and 1900 do not. The average year becomes 365.2425 days, closer to the tropical year's 365.2422 days.
Two problems drove reform. Julius Caesar's simple four-year leap rule overestimated the solar year by nearly one day per century, drifting the March equinox away from its nominal 21 March date—critical for calculating Easter. By 1582, that drift had reached ten days. Gregory's fix jumped from Thursday 4 October to Friday 15 October 1582, erasing the gap in one stroke while leaving weekdays intact.
The reform was Aloysius Lilius's proposal, refined by Christopher Clavius in an 800-page defence. Lilius also solved a long-standing Easter calculation problem by adjusting lunar tables. Catholic Europe adopted first; Protestant and Orthodox regions followed over three centuries, with Greece last in 1923 for civil use. Many Orthodox churches still use the Julian calendar for religious feasts.
Calendar cycles repeat every 400 years—146,097 days containing 97 leap years and 20,871 weeks. The mean year length is 365 days, 5 hours, 49 minutes, and 12 seconds. Historically, leap day was inserted by doubling 24 February rather than adding 29 February. The Council of Trent had authorized reform in 1545; decades of expert review followed before Gregory implemented it.
Source: Gregorian calendar
