Happy Leap Day

What is a Leap Year?
A leap year has 366 days, it has an additional single day to an ordinary year which has 365 days.

When do we have a Leap Day? 
During Leap Year, an extra day is added and  occurs every 29th of February. Nearly every 4 years is a Leap Year in our modern Gregorian Calendar. So we'll expect to have it on year 2016.


Do we need Leap Years?
Leap Years are needed to keep our 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 (a tropical year) – to circle once around the Sun.

Note: The illustration is not to scale.
However, the Gregorian calendar has only 365 days in a year, so if we didn't add a day on February 29 nearly every 4 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!

How do we calculate Leap Years?
In the Gregorian calendar 3 criteria must be met to be a leap year: 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, while1800, 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 over 2000 years ago, but the Julian calendar had only one rule: any year evenly divisible by 4 would be a leap year. This lead to way too many leap years, but didn't get corrected until the introduction of the Gregorian Calendar more than 1500 years later.

Source: timeanddate.com