Today is **February 29th**, **Happy Leap Year Day**! ..the day we get at the end of February every 4 years. February would have 29 days in a leap year instead of the usual 28.

Today, Leap Year Babies Can Celebrate their Birthday today after 4 years.

**What is a leap year?**

A leap year is a year in which one extra day has been inserted, or intercalated, at the end of February. A leap year consists of 366 days, whereas other years, called common years, have 365 days.

**Which years are leap years?**

In the Gregorian calendar, the calendar used by most modern countries, the following three criteria determine which years will be leap years:

1. Every year that is divisible by four is a leap year;

2. of those years, if it can be divided by 100, it is NOT a leap year, unless

3. the year is divisible by 400. Then it is a leap year.

**Understand why we have “leap years**

Leap years are years with **366 days**, instead of the usual **365**. Leap years are necessary because the actual length of a year is 365.242 days, not 365 days, as commonly stated. Basically, leap years occur every **4 years**, and years that are evenly divisible by 4 (2004, for example) have 366 days. This extra day is added to the calendar on February 29th.

However, there is one exception to the leap year rule involving century years, like the year **1900**. Since the year is slightly less than 365.25 days long, adding an extra day every 4 years results in about 3 extra days being added over a period of 400 years. For this reason, only **1 out of every 4 century** years is considered as a leap year. Century years are only considered as leap years if they are evenly divisible by 400. Therefore, **1700, 1800, 1900 were not leap years,** and 2100 will not be a leap year. But 1600 and 2000 were leap years, because those year numbers are evenly divisible by 400.