In the year 1582, 4th Oct was followed by 15th Oct - The Story behind those 10 Missing Days
By the 16th century, this drift had shifted the equinox by about 10 days, which affected the timing of religious observances like Easter. To correct this, Pope Gregory XIII issued a papal bull in february 1582, ordering the adjustment of the calendar. To bring the dates back in line, 10 days were removed, and the calendar was reformed to prevent further drift by altering the leap year rule.
The change was initially adopted by Catholic countries like Italy, Spain, and Portugal, but Protestant and Orthodox nations were slower to adopt it, leading to calendar differences between countries for centuries. The "10 missing days" represent a pivotal moment in timekeeping history, data-aligning the calendar with astronomical realities.