Leap Year

Taken from Writer's Almanac:

Today is Leap Year Day, when the month of February has 29 days instead of 28. This happens every four years and only in years that are divisible by four, such as 2004, 2008, 2012, and 2016.
The length of time it takes the Earth to completely orbit the sun is 365 days and six hours. Most calendars only list 365 days. An extra 24 hours accumulates every four years, requiring an extra calendar day. If we didn't account for this extra day, eventually, we'd have Christmas in July.

The Egyptians were the first to calculate the need for this type of regulation, but it wasn't put into practice until Julius Caesar, the Roman dictator, consulted with the top astronomers of the day and in 46 B.C. began adding one day (known as an intercalary day, or "leap" day) every four years to make up for the discrepancy. At the time, the ancient Roman calendar system was based on a total of 355 days, a full 10 ¼ days shorter than a solar year. Not one to waste an opportunity, he also decided to rename "Quintilis," the fifth month of the year (counting from March), which is how we got the month named "July."

Unfortunately, Caesar's new calendar system wasn't strictly enforced, and by the 16th century, it was almost 10 days off track, so in 1582, Pope Gregory reformed the Julian calendar. The calendar system we now use is called the Gregorian calendar.
According to British historical tradition, a leap day is the only day of the year a woman can propose marriage to a man.