Italy, Spain, Portugal and Poland all adopted the Gregorian calendar in 1582 -- eliminating all the days from October 5 and October 14 from the calendars in those countries.
The Gregorian calendar is so called because it was promulgated during the papacy of Gregory XIII. Wikipedia tells us that, since classical times, an 11 minute per year discrepancy between the true solar year and the old Julian calendar had resulted, by 1582, in the Spring equinox occurring on March 11 not on March 21. Given that the import of the celebration of Easter in Catholicism -- and given that Easter is supposed to fall on the first Sunday after the first full moon after the vernal equinox -- this had become an embarrassment.
Because the Pope was involved, Protestant Europe was slow to adopt the new calendar -- Great Britain and its possessions did not adopt the Gregorian calendar until 1752, during George Washington's lifetime. By that time, the gap was 11 days. (Yes, according to the calendar on the wall of his mother's house, little George would have been born on February 11, 1732, not February 22.)
Russia did not adopt the Gregorian calendar until after the Bolshevik Revolution. So when America acquired Alaska from Russia in 1867 (Seward's Folly, remember?) Friday, October 6, 1867 was followed again by Friday, October 18 (there were consecutive Fridays because the International Date Line was also shifted at this time, from Alaska's eastern border to its western one). Scientists are still attempting to calculate the role this confusion played in the development of Sarah Palin.
Today is also National Sausage Pizza Day. But it wasn't celebrated, even in Italy, on this day in 1582.