While many calendars throughout history show evidence of leap year-like adjustments, the modern leap day is credited to Julius Caesar, according to the BBC. 

Developed with the help of Egyptian astronomers, Caesar rolled out his eponymous Julian calendar around the year 46 BCE, primarily to address changes in the seasons being caused by earlier calendar inaccuracies.

Due to the slight differences between the 355-day calendar being used at the time and the actual astronomical movements of the planet, people began noticing marked seasonal drift; in other words, weather patterns associated with winter and summer were not aligning with the calendar winter and summer, shifts which posed threats to the farming and agricultural industry, among others.

The revised timetable was calibrated to the solar year, thereby taking into account the additional quarter-day and in-turn addressing the economic consequences faced earlier. Leap year also helped consolidate the array of calendars being used across the Roman Empire.

The switch came with consequences. In order to resolve previous years of incorrect calendaring, Caesar declared that 46 BCE would act as a 445-day transitional year. This has become known as the Year of Confusion.

However, the lingering discrepancy between .25 and .24219 began adding up, and over time people once again began noticing a misalignment between the date and the seasons.

By the late 16th century, it is estimated that the calendar was off by as much as 10 days, making it difficult, for example, for the Catholic Church to schedule Easter. 

Enter Pope Gregory XIII.

What Pope Gregory’s “Gregorian calendar” lacked in a creative name, it made up for in astronomical accuracy. 

In order to resolve the slight overcorrection presented by rounding .24219 up to .25, the new Gregorian calendar modified the existing leap year schedule to exclude centurials (the first year of a new century, such as 1700 and 1800) except for those divisible by 400 (e.g. 1600 and 2000).

This explains why there was a leap day in 2000, but there will not be a leap day in 2100, 2200, nor 2300. 

In the end, the leap day became a cause for celebration from when it was established continuing today, many cultures carpe this diem.

Around the 5th Century in Ireland, one bachelorette named St. Brigid of Kildare complained that men took “too long to make honest women of their sweethearts,” inspiring St. Patrick to decree that leap days would be celebrated with a gender-role reversal where women could propose to men. 

This tradition has since made its way to the United States, where it is commonly known as “Sadie Hawkins Day.”

A newspaper in France named La Bougie du Sapeur only publishes on leap days, and daughters in Taiwan are known to serve their parents pig trotter noodles to address the superstition that the elderly may be more likely to die in leap years.

A small city in Northern Italy known as Residents of, Reggio Emilia, a small city in Northern Italy, believe leap years are lucky for whales based on the thought that whales only birth during leap years, according to BuzzFeed.

However, not all leap year celebrations are so optimistic. Scottish culture frowns on leap day, and it's believed that “leaplings” — babies born on a leap day — will have “a year of untold suffering.”

Anthony, Texas, known to some as the “Leap Year Capital of the World,” disagrees, and holds a two-day leap year festival with an exclusive barbecue dinner for leap day babies.

Whether the 366th day of the year is lucky or not, it offers astronomers and calendar nerds everywhere a chance to enjoy the intricacies of time and space. 

And for those who are still confused or frustrated with the extra day, perhaps give a thought to the poor folks who had to deal with the extra 80 leap days during the Year of Confusion. 

Happy Leap Day to all, and to all a good leap year!

Chase McCleary is a multimedia journalist at Rocky Mountain PBS. Chasemccleary@rmpbs.org.