|Shifting of the date of the summer solstice|
from Absolute Astronomy
Every four years we add a day to our calendar: 29 February. We call this a "leap year," or an "intercalary year," or a "bissextile year." A year that is not a leap year is called a "common year."
This is necessary in order to keep our calendar (Gregorian) in sync with the seasonal year because the day is not exactly 24 hours long and extra time accumulates every year at the rate of about 1/4 of a day. In other words, our year is not 365 days, it's 365.25 days long. In order to keep things accurate, we add an entire day every fourth year.
Side note: On a much smaller scale, we also add leap seconds occasionally because the solar day becomes 1.7 milliseconds longer each century due to tidal friction. This information is tracked with atomic clocks by the International Earth Rotation and Reference Systems Service (IERS). It will be a very, very long time before we have a 366-day year (billions of years).
"Leap year: 2012 is a Leap Year" (timeanddate.com)
"Why we have leap days" (Bad Astronomy/Discover)
"LeapYear" (Wolfram Research)
"Leap years and leap seconds" (Royal Museums Greenwich, UK)
"Leap Years" (United States Naval Observatory (USNO))
General information & traditions (Wikipedia)
Galileo Day Campaign (Somnabulist)
The Galileo Project