Why do we have an extra day every four years?

We add an extra day to the calendar, known as a leap day, every four years to compensate for the fact that the Earth's orbit around the sun takes approximately 365.25 days. Without this adjustment, our calendar would gradually fall out of sync with the solar year. To account for this extra fraction of a day, we introduce a leap year with an extra day, February 29th, every four years. This practice is called the Gregorian calendar, named after Pope Gregory XIII, who introduced it in 1582 to align the calendar year more closely with the solar year. However, it's not a perfect solution, as the calendar still slightly overcompensates for the actual length of the solar year. To account for this, leap years are skipped in years divisible by 100 but not divisible by 400. This complex system helps maintain the accuracy of our calendar system over the long term.