When Pope Gregory XIII introduced the Gregorian calendar, he skipped 10 days to correct inaccuracies in the Julian calendar. This meant that October 4, 1582, was immediately followed by October 15, 1582, in countries that adopted the reform.
The Julian calendar, introduced by Julius Caesar, overestimated the length of the solar year by 11 minutes each year. This small error accumulated over centuries, causing the calendar to drift significantly from the equinoxes.
The French Republican Calendar, used during the French Revolution, divided the year into 12 months of 30 days each and used a 10-day week system called "decades." This radical departure from the Gregorian calendar was part of a broader effort to decimalise time.
The Mayan Long Count calendar was designed to track longer periods and ended a major cycle on December 21, 2012. This end date led to widespread but unfounded apocalypse predictions, highlighting the Mayan calendar’s complexity and cultural significance.
The Islamic calendar is purely lunar, consisting of 12 months that total about 354 or 355 days a year. Unlike solar calendars, it doesn't align with the Gregorian calendar, causing Islamic holidays to shift annually relative to the solar year and resulting in different seasonal contexts each year.