When the Gregorian calendar was adopted in 1582, 10 days were skipped to realign the calendar with the solar year. October 4, 1582 was followed directly by October 15, 1582 - so October 5-14, 1582 never existed in countries that adopted the calendar immediately (Italy, Poland, Spain, Portugal). September 14, 1852 existed, and February 29 existed in both 1804 and 1904 (leap years).