While most people would typically say that every year has just 365 days in it, that isn’t exactly true. In fact, it takes exactly 365 and one-fourth days for the Earth to travel around the sun. This means, that for the years to be entirely accurate and precise, it is necessary to have a “correction day” every four years, where an extra day is added to account for the extra one-fourth of a day.
The history of calendars has been a bit complicated with the Roman calendar including only 355 days. Reform of the calendar during Caesar’s time led to adding a “leap day”, but it was still a bit confusing. By the late 1500s, the Gregorian calendar (initiated by Pope Gregory XIII) began its movement into the modern world, and its practice of having an extra day every four years was slowly adopted and became common practice in the West over the next two centuries.
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