The leap year was introduced by Julius Caesar in 46BC, to make the calendar tidier. The extra day every fourth year made the average year 365.25 days long.
This was still about 12 minutes longer than the solar year, which you can get away with on the short term, but in 1267 a monk called Roger Bacon noticed that the calendar had slipped nine days in the 13 intervening centuries.
It then took the church until 1582 to accept that it was celebrating Easter on the wrong week. That year Pope Gregory XIII adjusted the calendar, introducing the system we go by today: every fourth year is a leap year, unless it is divisible by 100 and not by 400. This makes the year 365.2425 days, which is still a little under 26 seconds too long, but nothing to fret about.
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