Gregorian Calendar Reform

On February 24, 1582, Pope Gregory XIII issued papal bull 'Inter gravissimas' reforming the Julian calendar. The reform, based on proposals by Italian physician Aloysius Lilius with modifications by Jesuit mathematician Christopher Clavius, addressed two issues: ten days were removed to realign the vernal equinox (October 4, 1582 was followed by October 15, 1582), and a refined leap-year rule was implemented where century years would be leap years only if divisible by 400. Catholic nations adopted the reform immediately, while Protestant countries resisted for decades or centuries, creating a dual calendar system across Europe.