Today, we are going to talk about February 29. The extra day occurs once every four years. It is called a Leap Day and is added to the calendar. When there is a leap year there are 366 days rather than 365 days of the year.
The previous leap year was in 2012. This year sees another leap year. The extra day is added because of the way the Solar System works. A complete orbit of the Earth around the sun actually takes approximately 365.2422 days.
Why are there 29 days in February rather than 30 or 31? This is down to history. During the Roman Empire Emperor Julius Caesar ordered his astronomer Sosigenes to simplify the 355 day Roman calendar, which had an extra 22 days added on every two years. Sosigenes created the 365 day year, with an extra day every 4 years.