The Gregorian Calendar Reform

• Ten days were omitted from the calendar, and it was decreed that the day following (Thursday) October 4, 1582 (which is October 5, 1582, in the old calendar) would thenceforth be known as (Friday) October 15, 1582.

• The rule for leap years was changed. In the Julian Calendar a year is a leap year if it is divisible by 4. In the Gregorian Calendar a year is a leap year if either: 
• it is divisible by 4 but not by 100; or 
• it is divisible by 400. 

In other words, a year which is divisible by 4 is a leap year unless it is divisible by 100 but not by 400 (in which case it is not a leap year). Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not.

The Gregorian Calendar was adopted immediately upon the promulgation of Pope Gregory's decree in the Catholic countries of Italy, Spain, Portugal and Poland, and shortly thereafter in France and Luxembourg.

The Gregorian Calendar was adopted in Britain (and in the British colonies) in 1752, with (Wednesday) September 2, 1752, being followed immediately by (Thursday) September 14, 1752.

This led to the fact that Shakespeare and Cervantes both died on the same date - 23 April 1616. However due to the difference between the Julian and Gregorian calendars, their deaths were in fact 11 days apart.