Pope Gregory XIII introduces Gregorian calendar reform

The Gregorian calendar, introduced by Pope Gregory XIII via the papal bull ‘Inter Gravissimas’, corrected errors inherent in the Julian calendar to more accurately reflect the length of the tropical year. Following the reform, the day after October 4, 1582, became October 15, 1582, effectively omitting 10 days to realign the calendar.

The reform introduced a new leap year rule: years divisible by 100 are not leap years unless they are also divisible by 400. This adjustment changed the average year length to 365.2425 days. While Italy, Poland, Portugal, and Spain adopted the system immediately, other nations transitioned at different times over several centuries.

In the context of specific regions, parts of Croatia transitioned in 1587, while Bulgaria adopted the calendar via law in 1916.