February 29th occurs in leap years like 2012.

Leap years occur every four years, adding one extra day to the month of February. This brings the total days in a leap year to 366.

The day is added to bring the calendar into alignment with the earth's revolution around the sun, which actually takes about 6 hours more than the time for 365 days. Adding one day every fourth year reduces the difference between the calendar of 365 days and the actual length of a tropical year, or the time between two successive vernal equioxes. 
 

Xpeditions
In Seasons: Why It's Essential (9-12), students design their own monuments to keep track of the seasons and the passage of time.
 

Wonderopolis
Consider How many Seconds Are In a Year? (PK-6) in this Wonder of the Day that reviews how we measure the passage of time.

Find out more about the month we call February and the eleven others that form our yearly calendar in the Wonder of the Day: How Did the Months of the Year Get Their Names? (PK-6).