The Gregorian Calendar was introduced by Pope Gregory XIII to replace the Julian Calendar, which had been in use since its establishment by Julius Caesar in 45 B.C. The Julian system miscalculated the length of the solar year by about 11 minutes, resulting in a drift of the calendar with respect to the equinoxes over centuries. By the middle of the 16th century, this drift had caused significant discrepancies, particularly affecting the date of Easter celebrations, which depended on the timing of the spring equinox. To address this, Pope Gregory XIII convened a commission to study the calendar's flaws and propose a correction.

On October 4, 1582, countries that adopted the new calendar skipped directly to October 15, 1582, effectively removing 10 days from the calendar. This immediate adjustment was necessary to realign the equinoxes correctly. To prevent future errors, the new calendar established a revised system for leap years, introducing the rule that a year would be a leap year only if it was divisible by four, except for years that were divisible by 100 but not 400. Thus, the year 1600 was a leap year, but 1700, 1800, and 1900 were not.

The implementation of the Gregorian Calendar was not universally accepted, leading to confusion and a schism between countries that had adopted it and those that remained with the Julian Calendar. Catholic countries like Spain, Portugal, and Italy made the transition quickly, while Protestant nations, including England and its colonies, continued to use the Julian system until the 18th century. It wasn't until 1752 that Great Britain officially switched to the Gregorian Calendar, creating additional complexities in historical records.