A leap year is a year that contains an extra day, February 29, giving it 366 days instead of the usual 365. Leap years keep our calendar aligned with Earth's orbit around the Sun, which takes approximately 365.25 days to complete.
A year is a leap year if it is divisible by 4, except for century years (years ending in 00), which must be divisible by 400. So 2000 was a leap year, but 1900 was not. The next leap years are 2028, 2032, and 2036.
We have leap years because Earth takes approximately 365.25 days to orbit the Sun. Without adding an extra day every four years, our calendar would gradually drift out of sync with the seasons, eventually causing summer to fall in what we now call December.
People born on February 29 are sometimes called 'leaplings' or 'leap year babies.' They only experience their exact birth date every four years, so many celebrate on February 28 or March 1 in non-leap years. Famous leap day birthdays include composer Gioachino Rossini and motivational speaker Tony Robbins.
Century years are only leap years if they are divisible by 400. This is because adding a full extra day every 100 years would slightly over-correct the calendar drift. The year 2000 was a leap year (divisible by 400), but 1700, 1800, and 1900 were not.