Gregorian Calendar Takes Effect in Europe
By the late 16th century, the Julian calendar had drifted about 10 days behind the solar year due to inaccuracies in leap year calculations, affecting religious observances like Easter. Pope Gregory XIII issued a papal bull in 1582 ordering reform to realign the calendar with astronomical reality. In adopting Catholic countries including Italy, Spain, Portugal, and Poland, Thursday, October 4, 1582, became the last day under the Julian system. The following day was designated Friday, October 15, skipping the intervening dates. This adjustment corrected the drift and introduced refined leap year rules still used worldwide today.
Why it matters: The reform standardized timekeeping across much of Europe and eventually the globe, improving accuracy for agriculture, navigation, and ecclesiastical calculations. It resolved centuries of accumulating error and became the international civil calendar, influencing global coordination in science, commerce, and daily life.