The 1582 calendar is famous for the transition from the Julian to Gregorian calendar, enacted by Pope Gregory XIII. To correct the accumulated drift of the old calendar, 10 days were removed in October: Thursday, October 4th, 1582, was followed immediately by Friday, October 15th, 1582, in Catholic regions. Key Aspects of the 1582 Calendar Transition: Missing Days: October 5 to October 14, 1582, never occurred in regions adopting the change immediately, such as Italy, Spain, Portugal, and Poland. Reason for Change: The Julian calendar calculated the year as 365.25 days, which was slightly too long. This caused the spring equinox to drift from March 21, affecting the timing of Easter. Implementation: The reform was introduced by the papal bull Inter gravissimas on February 24, 1582. Delayed Adoption: While Catholic nations adopted it immediately, other regions (like England and Russia) did not switch until much later, resulting in different dates between countries in the interim. October 1582 Structure: Oct 1-4: Mon–Thu Oct 15-31: Fri–Wed (followed by November starting on a Thursday)
Note: The days 5-14 simply did not exist in this calendar.
