In Italy, Poland, Portugal and Spain, the day of October 5, 1582, does not exist because of the implementation of the Gregorian calendar made by Pope Gregory XIII.
For most of world’s countries using the Julian calendar: Friday 5 Oct 1582, was, however, a normal date.
In Italy, Spain, Portugal and Poland, this was the first of ten dates that were skipped with the introduction of the Gregorian calendar In those four countries.
Thus, Thursday Oct 4, 1582 was the last day the Julian calendar was used, and today’s date became Friday 15 Oct 1582 in Italy and the Catholic countries under the pope’s decree.
This historical and important change was introduced by Pope Gregory XIII to realign the calendar with the equinoxes, and the lunar cycles used to establish when to celebrate Easter. Many Protestant countries initially objected to adopting a Catholic implementation; some Protestants feared the new calendar was part of a plot to return them to the Catholic fold.
Britain, Ireland and the British colonies, including those on the East coast of America, resisted this Popish change, and used the Julian calendar for more than a century and a half until Sep 2, 1752.
The Gregorian calendar is a solar calendar. A regular Gregorian year consists of 365 days, but as in the Julian calendar, in a leap year, a leap day is added to February. In the Julian calendar a leap year occurs every 4 years, but the Gregorian calendar omits 3 leap days every 400 years.