Today is a day that happens once every four years with the leap year. The last time was in 2012, and the next time will be in 2020. Most people know that, but most don't know why we even have a leap year, or why in the world the leap day is always at the end of February.
The full reason we have a leap year and the need for a leap day is that our calendars and the actual rotation of the earth around the sun doesn't add up, astronomically speaking. One of the first things that children learn is that it takes 365 days for the earth to orbit the sun, but this isn't an exact whole number. The truth is that it takes 365.2422 days to complete, which doesn't match up with our Gregorian calendar. If you do the math and multiply this number by 4, you will see that the leap year is so our years will be more exact.
The development of the leap year goes as far back as the days of Julius Caesar. According to the Telegraph, the Roman calendar used to have 355 days with an extra 22-day month every two years. In the first century, Caesar ordered an Alexandrian astronomer Sosigenes to devise something that would work better. So Sosigenes created the 365-day year with an extra day every four years to incorporate the extra hours.
As for why this takes place at the end of February, it wasn't always this way. The third-century writer Censorinus says that "the intercalation was in preference made in February, between the 23rd and 24th". Part of the reason the end of February was eventually chosen was due to the fact that March 1 was the first day of the calendar year.
Some have attributed the choice of February to be leap day related to the ego of Caesar Augustus. Under Julius Caesar, the month of February had 30 days and the month named after the emperor, July, had 31 days. August at the time had 29 days, so Caesar Augustus added two days to the month named after him. So February lost out to August in the battle for extra days.
In 1582, Pope Gregory's astronomers decided to lose three days every 400 years as the new calendar was introduced. So far, the math has worked out well, but science shows that the system will need to be rethought in 10,000 years.
Technically, a leap year is not every four years. It is in years divisible by four, but not for years that are both divisible by 100 and not divisible by 400. That means that even though 2000 was a leap year, the years of 1700, 1800, and 1900 were not. The added rule of centuries is to make certain the extra decimals add up.