Why do we have leap years?

It would be 24th November 2019 if there were none. a) What are leap years? Every year that is exactly divisible by four is a leap year, EXCEPT for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are. So 3 leap years every 400 years are omitted. b) Ok, but why do we have them? Because Julius Caesar decided so. Why did he? To keep our calendars synchronised with the seasons, which is determined by the earth's orbit around the sun. c) Why do they need to be in sync with the seasons? One important Christian reason is to maintain the consistency of Easter. d) What's with that Centurian exception? Julius Caesar thought the earth's orbit took 365.25 days, but it actually takes a little less time: 365.2422 days, so his calendar (Julian) was about 11 minutes too long. So Pope Gregory 13th sees that the seasons are not synchronising, finds out why, and decides that 10 days in October 1582 will vanish, and that from then on, 3 leaps in every 400 years will be removed to compensate for that 11 minutes.

Leap years happen because the sun revolves around the earth in 365 and a quarter days. To correct for this difference and keep our calendars in sync with seasons, we add an extra day every 4 years: the 29th of February.