By the middle of the 16th century the Calendar used by the Christian Religious leaders, the Julian Calendar designed and adopted during the reign of Julius Ceasar in 45 BC, was off by around 10 days.
This was causing confusion on when Easter, as adopted by the First Council of Nicaea in 325, would be. Pope Gregory decided that calendar reform was needed. The outcome of this reform was our current calendar, also called the Gregorian Calendar.
Part of the reform was to add these 10 days to the calendar. This action was adopted on February 24, 1582. Later in the year on October 5th, five Catholic counties adopted the calendar. The day after October 5th was October 14th in these countries.
When a county added these days to the calendar it caused for these days to be skipped. England, for example, didn’t adopt the Gregorian Calendar until 1754. At that time another day needed to be added to the Calendar. September 2, 1752 was the last day that England observed the Julian Calendar with the next day being September 14th.
Adoption was slow, in fact as late as the 20th Century some countries were still on the Julian Calendar, including Russia, who changed in 1918, Greece in 1923 and Turkey in 1926.
Another change came in what was the beginning of the year. The Julian Calendar called March 1st as the first day of the year, while the Gregorian Calendar it became January 1st.