A leap year has 29 days in February, making the year 366 days long. The Earth takes about 365.2422 days to orbit the Sun, so fixing a year at 365 days creates an error of about 0.2422 days each year. To correct this drift, an extra day (February 29) is added every four years β that is a leap year. People born on February 29 only celebrate their actual birthday once every four years.
A leap year is determined by these rules: First, if the year is divisible by 4, it is a leap year (e.g., 2024). Second, however, if it is divisible by 100, it is a regular year (e.g., 1900). Third, but if it is divisible by 400, it is a leap year again (e.g., 2000). These are the Gregorian calendar leap year rules, which keep the average year length at 365.2425 days, minimizing the error against the actual astronomical year.
The concept of leap years began with the Julian calendar introduced by Julius Caesar in 46 BC. The Julian calendar added a leap year every four years but calculated the year as 365.25 days, slightly longer than reality. To fix the accumulated error, Pope Gregory XIII introduced the Gregorian calendar in 1582, adding the 100/400 exception rules to complete the leap year system we use today.