Gregorian calendar

Pope mandated a calendar switch, causing holidays to be more aligned with seasons. A leap year every 4 years means a year is 365.25 days, but it’s actually 365.2422 days long. To avoid error new calendar introduced a new rule. Instead of 1/4, 97/100 had to be leap years. To adjust for the offset, 10 days were dropped on October 4, 1582. The next day was October 15.
 Christoph Clavius Rome 1582 Gregorian calendar

Pope mandated a calendar switch, causing holidays to be more aligned with seasons. A leap year every 4 years means a year is 365.25 days, but it’s actually 365.2422 days long. To avoid error new calendar introduced a new rule. Instead of 1/4, 97/100 had to be leap years. To adjust for the offset, 10 days were dropped on October 4, 1582. The next day was October 15.