Just to replace a bit the British bias by a broader perspective: The Gregorian Calendar was introduced in Italy, Spain, Portugal and catholic Poland in October, 1582, when October, 15th, followed October 4th (and was soon adopted by the other Roman Catholic countries). 170 years later, in 1752, the Gregorian calendar was finally adopted in Britain, not in October however, but in September. For coming late, the British had to pay dearly (grin): they now lost eleven days instead of ten (for the two calendars disagreed on whether 1700 was a leap year or not).

The British were not the only to come late (actually they were among the first of the not catholic countries to comply): The first mostly protestant country at all to introduce the Gregorian calendar was Prussia in 1610. The other protestant parts of Germany adopted the reform in 1700 (at the latest possible date to loose only 10 days) to end a long period of confusion due to two different calendars in Germany, Sweden came in 1753, Japan in 1873, Russia only after the October revolution (which was actually in November) in 1918 (most orthodox countries also followed in the twentieth century) and China in 1949 (another source: 1911).

To add a bit to what Wandering Minstrel has written about the reluctance of the people to accept the new calendar. Early environmentalists even reproached the pope with disturbing the migration of the birds and thus causing their deaths. If people emotionally oppose a change no argument is too stupid not to be used (no, I'm not going to link to the thread about the good old British money).

Wolfgang