Well, it usually comes every four years. Turns out there are exceptions, and I'm not sure I knew about them.
I give full credit to the folks over at TimeAndDate.com, which is the place from which I stole the following material. I just thought it was too good not to share:
Why do we have Leap Years?
Leap years are needed to keep our modern day Gregorian Calendar in alignment with the Earth's revolutions around the sun. It takes the Earth approximately 365.242199 days – or 365 days, 5 hours, 48 minutes, and 46 seconds – to circle once around the Sun. This is called a tropical year.However, the Gregorian calendar has only 365 days in a year, so if we didn't add a day on February 29 nearly every four years, we would lose almost six hours off our calendar every year. After only 100 years, our calendar would be off by approximately 24 days.
Which Years are Leap Years?
In the Gregorian calendar, three criteria must be taken into account to identify leap years:
- The year is evenly divisible by 4;
- If the year can be evenly divided by 100, it is NOT a leap year, unless;
- The year is also evenly divisible by 400. Then it is a leap year.
This means that 2000 and 2400 are leap years, while 1800, 1900, 2100, 2200, 2300 and 2500 are NOT leap years.
The year 2000 was somewhat special as it was the first instance when the third criterion was used in most parts of the world since the transition from the Julian to the Gregorian Calendar.
Who invented Leap Years?
Julius Caesar introduced Leap Years in the Roman Empire more than 2000 years ago, but the Julian Calendar had only one rule: any year evenly divisible by 4 would be a leap year. This led to way too many leap years, but it didn't get corrected until the introduction of the Gregorian Calendar more than 1500 years later.
