Did You Know That October 5, 1582 Did Not Happen? Yes, as well as October 6-14 of the same year.
To fully understand the story, let’s take a few steps back in time first:
In the early times, the Sumerians of Babylonia used the phases of the moon to determine the dates, counting up to 12 lunar months a year. To make up for the difference between the lunar system and their year of seasons, they inserted an extra month in their calendar about every four years.
Eventually, as most trends in history, some societies copied the Summerians. The Egyptians, Greeks and Semitic people adapted the Summerian calendar. Not long after that, the Egyptians developed their own system that was almost accurate to the seasons.
Meanwhile, in Rome, they were using a calendars which was also based on the moon (which was probably borrowed from the Greeks who copied from the Summerians). The year in their calendar was 355 days long. The Roman months of corresponding to March, May, July and October had 31 days each and the rest had 29. An extra month was also added every four years.
The great Roman society assigned the regulation duties for this calendars to high priests. Every calend (or day of the full moon) priests announces to the people the number of nones (quarter moons) and ides (full moons) for that month, hence the root of the word calendar,the Latin kalendae.
However, due to some reasons, the priests had performed their duties poorly putting summer months during spring. This ticked of Julius Caesar (imagine how pissed he was) and ordered a revision. He adapted the plan of an Egyptian mathematician Siogenes. The new calendar will be composed of 365 days every year and will be adding a day every fourth (or called leap) year. Caesar also distributed the extra ten days among the 29-day months making them identical with the months we know today.
He also named the month Quintilis to July (in honor of him) and the month Sextilis to August (in honor of the emperor Augustus).
Some stories say that Emperor Augustus ordered that his month be changed by adding one more day so that it would be as long as Caesar (31). This, however, was not proven.
The correction of Caesar on the leap years made the calendar longer than the year of seasons. Thus, anniversaries began coming earlier and earlier in the year. To wit, in 1582, the Vernal Equinox, which signaled the beginning of spring, occurred in March 11 instead of 21.
Of course, this had significance for the Catholic Church, which regularly celebrated Easter on the first full moon of the equinox, which was pre-fixed by the church at March 21. The mis-calculation of dates will lead to errors in major religious holidays.
A Jesuit mathematician and astronomer named Christopher Clavius, suggested that the Julian calendar be changed to fix this error. This will result to a drop in a leap day three times in a 400 year cycle. Every year that is divisible by 100 and not by 400 will lose its leap day. This resulted to a 365.2454-day year, which was nearly accurate as the average solar year of 365.2424 days.
To restore the original day of the equinox, Clavius endorsed a 10-day skip for October 5-14 of 1582. The major Catholic nations in Europe followed the order, while some Protestants followed the next years. Case in point, Russia, Greece and other Easter European countries still used the Julian calendar up to the 20th century.
Today, the fifth through 14th day of October in 1582, is, as recorded in history, in-existent.
Sources: Compton’s Encylopedia, 1999