One year has the length of 365 days, 5 hours, 48 minutes and 45 seconds. This is hard to calculate with, so a normal year has been given 365 days and a leap year 366 days. In leap years, February 29th is added as leap day, which doesn't exist in a normal year. A leap year is every 4 years, but not every 100 years, then again every 400 years.
This year 2024 is a leap year. The last leap year was 2020, the next will be 2028.
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The fact that the length of a year is not divisible by the length of a day without a remainder was already known in ancient times. To solve this problem, Julius Caesar set the leap day on February 29th in 45 BCE when he introduced the Julian calendar. This improved the calendar enormously and pushed the problem of days not matching exactly several centuries into the future. But a quarter of a day (six hours), which is delayed every four years by the leap day, is a little more than 5 hours, 48 minutes and 45 seconds. And at some point the error became apparent in that the days of the year were at a different point in the seasonal cycle. The Julian calendar was one day too slow every 128 years. In 1582, Pope Gregory XIII introduced the Gregorian calendar, and in that year the 4th was followed by the 15th of October, making up ten days. It took a good 200 years for it to become accepted worldwide. The solution that every 100 years is not a leap year, but every 400 years is, represented the true length of the day in a year fairly well, but not completely accurately. The Gregorian calendar is one day too slow every 3231 years, so in the distant future a day will have to be skipped in the calendar. This will probably be a leap day that then will not occur.