We are all used to ever 4thyear having an extra day, and having to work an extra day unpaid! But how and why did the tradition begin and why is the extra day added to February. The simple answer is that the movements of our solar system mean that a complete solar year takes 365.2422 days.
To account for that extra .2422 of a day, every 4 years an extra day is added. However that isn’t the end of the story.
The leap year’s extra day is necessary because of the “messiness” of our Solar System. One Earth year (a complete orbit around the Sun) does not take an exact number of whole days (one complete spin of the Earth on its axis). In fact, it takes 365.2422 days, give or take. Because the exact amount is slightly less than a quarter of day, we cannot add an extra day every 4 years without fail. As a result 3 leap days in every 400 have to be skipped 8the next three will be in 2100, 2200 and 2300.
Pope Gregory XIII and his astronomers were responsible for the system, when they introduced the Gregorian calendar in 1582. It will need to be rethought in about 10,000 years’ time, unless we’ve come up with an alternative by then!
The reason for the extra day being added in February comes down to the Roman Emperor Caesar Augustus. Under Julius Caesar, February had 30 days, but when Caesar Augustus was emperor he was upset that the month named after him (August) only had 29 days, but Julius Caesar’s month (July) had 31, so he took 2 days from February and added them to August.
Other calendars apart from the Gregorian require leap years. The modern Iranian calendar is a solar calendar with eight leap days inserted into a 33-year cycle. The Indian National Calendar and the Revised Bangla Calendar of Bangladesh arrange their leap years so that the leap day is always close to 29 February in the Gregorian calendar.