The rule
A year is a leap year if it can be divided by 4 — except century years (1900, 2100), which are leap years only if they can be divided by 400. So 2000 was a leap year, but 1900 was not and 2100 will not be.
That gives 97 leap years every 400 years, and an average calendar year of 365.2425 days. The solar year that sets the seasons is about 365.24219 days, so the calendar gains only one day on the seasons every 3,200 years or so.
Why the century rule?
Julius Caesar's calendar, which took effect in 45 BC, had a leap year every fourth year without exception — an average of 365.25 days. That is about 11 minutes too long, and over the centuries the error added up: by the 1500s the spring equinox had crept from 21 March to around 11 March.
In 1582 Pope Gregory XIII brought in the reform we still use. It removed ten days at once — in the countries that adopted it first, Thursday 4 October 1582 was followed by Friday 15 October — and added the century rule so the drift would not return. Britain and its colonies, including the American colonies, switched in 1752, when Wednesday 2 September was followed by Thursday 14 September. Russia changed in 1918 and Greece in 1923.
The next leap years
2028, 2032, 2036, 2040, 2044 and 2048.
Born on 29 February?
A leap day comes round once every 1,461 days, so about one person in 1,461 is born on it. In years without a 29 February, most people celebrate on 28 February or 1 March. Our age calculator treats the birthday as 28 February in those years.