What Calendar Was Used In Europe Before the Gregorian Calendar?

The answer is: The Julian Calendar

The Julian calendar had a good run for about 1,600 years, but it had one tiny flaw that caused massive headaches. Julius Caesar introduced it in 45 BCE after consulting with Alexandria's top astronomer, Sosigenes. They calculated each year at exactly 365.25 days, which seemed perfect. Just add a leap day every four years and you're golden, right?

Not quite. The actual length of a solar year is 365.242190 days. That .0078 day difference doesn't sound like much, but it adds up. By 1582, the calendar was 10 days behind the actual solar year. Pope Gregory XIII was not having it, especially since Easter was getting seriously out of sync with spring.

Gregory's team of astronomers created a modified system: skip the leap year in century years unless they're divisible by 400. October 4, 1582, was followed by October 15 to fix the accumulated error. Catholic countries jumped on board immediately. Protestant nations took centuries to adopt it because they didn't want to follow the Pope's calendar. Britain and its colonies didn't switch until 1752 when they had to just delete 11 days. And Russia held out until after the 1917 revolution, which is why their October Revolution happened in our November.