Every four years during the second month of the year, February is gifted with an extra day. Although this extra leap day is well known, the history behind the start of this tradition is something most people are unfamiliar with. 

According to the Encyclopedia Britannica, around 738 BCE the calendar that was used contained 304 days. This Roman calendar caused many issues pertaining to the alignment of the seasons with the solar year. As a result, the Romans decided to occasionally add an extra month, called Mercedonious, to realign the seasons with the solar year. The Roman calendar then contained 355 days.

This solution was short-lived and around 46 BCE, the Roman calendar was three months ahead of the solar year. Julius Caesar, who was the ruler at this time, was advised to implement the Egyptian calendar, which contained 365 ¼ days. This added two months to the Roman calendar and every month would contain 30 or 31 days except February, which would contain 28. This was not the final solution, because Caesar also decided every four years would have an extra day in February, which birthed the start of leap years. Our current calendar is now considered the Julian calendar.

By 1582, the Julian calendar had caused a gap of 10 days between the solar year and the Julian calendar. Astronomers discovered a single year contained 365 days, 5 hours, 48 minutes and 46 seconds instead of 365 ¼ days. Pope George Xlll moved the calendar up 11 days to correct the 10-day error and established a new calendar. The calendar would be structured the same as the Julian calendar, but the leap years would only be every four years that are divisible by 400. This causes century years, like 1900, to be removed from being a leap year because it is not divisible by 400. This solution was established after astronomers discovered that three extra days would be accumulated every 400 years.

According to History.com, the modern calendar used today, although fixed from past errors, is still not completely accurate. Advancements in technology have allowed for the ability to hone the accuracy of the modern calendar and resulted in the discovery that the modern calendar has about a 26-second difference from the solar year. This small error will take 3,000 years to be noticeable, but solutions will be found to correct the calendar.