Did you know that in the year 1582, Oct. 4 was followed directly by Oct. 15?
In ancient times, people used to keep time and days using moon cycles. However, this system did not keep the year in tune with the seasons. Every once in a while an extra month had to added to align with the seasons.
In 46 BC, Julius Caesar noticed that Egyptians used a calendar based on the sun. He created his own system, the Roman or Julian calendar, to solve the problem of the drifting seasons. Each solar year in Caesar's calendar had 365 days and an additional six hours. He adjusted this by adding an extra day (leap day) every four years, to the shortest month.
However there was one small problem. Caesar's estimation of a year was off by 11 minutes every year. By 1582, this had accumulated to a 10 day discrepancy. It would affect Christian festivals such as Easter that was set by the moon. Pope Gregory XIII decided to fix the issue. He issued a Papal bulletin by which Oct 4 was followed directly by Oct 15 and the time in between was erased on the calendar and declared non-existent by the Pope.
To fix the problem for the future, he further declared that every fourth year would be leap year, except if the year happened to be the beginning of a century. However, every 4th century (400 years) would still be a leap year.
This calendar became popular as the Gregorian calendar. It is widely used all over the world. Google is celebrating the 434th anniversary of the Gregorian calendar with a doodle.