The twenty-ninth day of February comes only once every four years. The reason this occurs is due to a quirk of the Earth's orbit and the limitations of the human calendar system: In extremely simplified terms, the Earth's orbit around the sun isn't completed in a perfect 365 days. There's an extra quarter of a day that isn't accounted for by our calendars, so once every four years,note we add an extra day to the calendar to balance things out.
People born on this day are known as "leapers" or "leaplings" and when it comes to acknowledging their day of birth, leapers are in a bit of a bind given that their day doesn't technically exist most years. One way to get around this is by celebrating the day before or after. This is what legal jurisdictions do for the purposes of determining when someone reaches the age of majority/legal adulthood. The United Kingdom and Hong Kong use March 1, while New Zealand and Taiwan use February 28. In the United States, this is typically determined by state statute.
As the only date in the calendar that doesn't occur in every year, it has some comedic potential, especially when it's someone's birthday: Any event happening on Bob's "Xth birthday" won't actually happen until he's 4 times X years old.note