A leap year is a year that has one extra day added to it, making it 366 days long instead of the usual 365 days. This extra day is added to the month of February, which has 29 days in a leap year instead of 28. Leap years occur every four years to help synchronize the calendar year with the solar year, which is about 365.25 days long. To determine if a year is a leap year, you can follow a simple rule: if the year is divisible by 4, it is a leap year, unless it is also divisible by 100. However, if it is divisible by 400, it is still a leap year. For example, the year 2000 was a leap year, but 1900 was not.