The Ten Days That Never Happened
In October 1582, Thursday the 4th was followed directly by Friday the 15th. The days in between were removed by decree, through the papal bull *Inter gravissimas* issued by Pope Gregory XIII.
This was not a whim. Those ten days were a debt that had been accumulating for over a thousand years, and cancelling them was the only way to settle it. The bull replaced the Julian calendar with what became the Gregorian calendar, now used in most of the world.
The reason was religious arithmetic. The date of Easter is calculated from the spring equinox, which the Church had pegged to 21 March. By 1582 the actual equinox was arriving well before that nominal date, so the calculation was steadily drifting away from reality. The reform pulled the equinox back to where it had sat at the First Council of Nicaea in AD 325, and also corrected the lunar cycle used for Easter, because astronomical new moons were occurring four days before the calculated ones.
A Calendar Losing a Day a Century
The Julian calendar assumed the average solar year is exactly 365.25 days long, and so added a leap day every four years without exception. It was very nearly right, which is precisely what made it dangerous.
The true tropical year is about 365.2422 days. The overestimate is a little under one day per century, small enough to ignore in a lifetime and impossible to ignore across a millennium.
European scholars had known about the drift since the early medieval period, and watched it grow. Bede, writing in the 8th century, showed the accumulated error was already more than three days. Roger Bacon estimated seven or eight days around 1200. Dante was aware of the need for reform around 1300. Pope Sixtus IV invited the astronomer Regiomontanus to the Vatican in 1475 to fix it, but the project collapsed when Regiomontanus died shortly after arriving in Rome. It would be another century before anyone finished the job.
Skip Three Leap Days Every Four Centuries
The actual fix is elegantly small. Every year divisible by four is a leap year, except years divisible by 100 that are not also divisible by 400.
That is why 1800 and 1900 were ordinary years while 1600 and 2000 were leap years, a detail that quietly trips up date arithmetic to this day. The rule removes three leap days from every four centuries, shortening the average calendar year by 0.0075 days and bringing it to 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds. That is within about half a minute of the real tropical year.
The scheme came from the Calabrian doctor Aloysius Lilius, with the German mathematician Christopher Clavius as the architect of the finished calendar. The result is beautifully periodic: the whole cycle repeats exactly every 400 years, or 146,097 days, made of 303 ordinary years and 97 leap years, which works out to exactly 20,871 weeks.
Three Centuries of Refusing
Adoption split along religious lines almost immediately. The Catholic countries of Europe and their overseas possessions switched first. Protestant and Eastern Orthodox countries moved gradually over the following three centuries, generally declining to call it Gregorian and referring instead to the "Improved calendar".
The delay was long enough to make historical dates genuinely ambiguous. Documents from the transition period are tagged "Old Style" or "New Style" so readers know which calendar a date belongs to, a convention historians still use.
Greece was the last European country to adopt it, in 1923, and then only for civil use. Many Orthodox churches continue to use the Julian calendar for religious rites and the dating of major feasts, which is why some Christmas and Easter dates still differ. Most non-Western countries adopted the calendar during the 20th century, again generally for civil purposes rather than religious ones.
