The Gregorian calendar was first adopted in Italy, Poland, Portugal and Spain in 1582.
The Gregorian reform consisted of the following changes:
In September 1752, Great Britain switched from the Julian Calendar to the Gregorian Calendar.
In order to achieve the change, 11 days were "omitted" from the calendar - i.e. the day after 2nd September, 1752 became the 14th September, 1752.
However, many countries used the Julian Calendar much longer.
Turkey was the last country to officially switch to the new system on 1st January, 1927.