1582
The proclamation of the Gregorian calendar
Computing the length of the solar year is a tricky proposition for any civilization but especially for those without any of the astronomical tools we possess today. In 46 BC Julius Caesar reformed the calendar by decreeing a year of 365 days with a leap year every fourth year. This was an improvement but resulted in a difference of 3 days every four centuries between the calendar and the solar year. By the 16th century the slippage was notable, causing Easter to be divorced from the spring equinox and disrupting traditional agricultural practices which were based on saints’ days.
In 1582 Pope Gregory XIII, after taking advice from leading astronomers, added a further reform to the Julian calendar:
Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. (This meant that 1900 was not a leap year but the year 2000 was.)
In addition, the Pope decreed, in order to make up for the 1,600 years of accumulated error, that 10 days would be skipped. The Julian calendar day Thursday, 4 October 1582 was followed by the first day of the Gregorian calendar, Friday, 15 October 1582. This caused considerable popular discontent as many of the mathematically-challenged peasantry felt they had been robbed of a chunk of their life. It has caused head scratching for historians as well because much of Europe — those parts with Protestant and Orthodox churches — did not adopt the Catholic pontiff’s decision, making dating documents troublesome in retrospect. This is why readers will sometimes see some early-modern British dates referred to as “O.S.” (Old Style) and “N.S. (“New Style”). Britain do not switch until 1752 and it was not until the 20th century that Greece, Turkey and the Soviet Union adopted the Gregorian calendar. The Orthodox Church still uses the Julian calendar which will explain Western Canadians referring to “Ukrainian Christmas” occurring on January 7th.