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The calendar we use today, the Gregorian calendar, has a long and fascinating history. It's a refinement of the Julian calendar, which was introduced by Julius Caesar in 45 B.C. The Julian calendar was a significant improvement over the previous Roman calendar, but it had a small error: it overestimated the length of the year by about 11 minutes. Over centuries, this small error caused the calendar to drift significantly out of sync with the seasons.

By the 16th century, the vernal equinox was occurring 10 days earlier than the calendar said it should. To correct this, Pope Gregory XIII introduced the Gregorian calendar in 1582. He skipped 10 days to realign the calendar with the seasons and established a more accurate leap year rule: a leap year occurs every 4 years, except for years divisible by 100 but not by 400. This is the system that most of the world uses today.