From the Math Forum Newsletter: What are leap years?
2012 is a leap year, so this month ends on Wednesday, February 29th.
In the leap year of 1996, the Forum began a free ask-an-expert service for math students and their teachers. Since then, Ask Dr. Math has helped millions understand why some years have 366 days, plus lots more mathematics about calendars, including
Find out the answers at http://mathforum.org/dr.math/faq/faq.calendar.html