Leap Day

A Leap Day, February 29, is added to the calendar during leap years. This extra day makes the year 366 days long – not 365 days, like a common year.

 

When Is the Next Leap Day?

2016 is a leap year, so the next leap day is
February 29, 2016.
The last Leap Day was on February 29, 2012.

Why Add a Leap Day?

Leap days are needed to keep our calendar in alignment with the Earth's revolutions around the Sun.
It takes the Earth approximately 365.242189 days – or 365 days, 5 hours, 48 minutes, and 45 seconds – to circle once around the Sun. This called a tropical year.
Without an extra – or intercalary – day on February 29 nearly every four years, we would lose almost six hours every year. After only 100 years, our calendar would be off by approximately 24 days in relation to fixed seasonal days days like the vernal equinox or winter solstice.
More info at
http://www.timeanddate.com/date/leap-day.html