"Give us the eleven days back!"
On September 2, 1752, the British finally switched to the Gregorian calendar, synchronizing with the Catholic nations in Europe.
September 3rd did not happen. Neither did September 4th or 5th. In fact, by official decree, September 3rd became September 14th. Humans have a charming myth allowing them to follow the calendar and the clock religiously, attaching real significance to the numbers and names associated with specific days.
The result? Pandemonium! Riots broke out as the people demanded their days back; people felt they had lost time.
(Liveforever makes an excellent point--the riots had less to do with metaphysical musings than the very real loss of income. Rent was still due, and the opportunity to earn money had been reduced. People felt a very real financial burden imposed by this decree.)
The Gregorian calendar had already been adopted by much of Europe on October 4, 1582, when Pope Gregory XIII declared that day to be October 15, 1582. The new calendar was based on a tropical year, about 365 1/4 days, the time it takes the Earth to orbit about the sun.
The actual credit for the calendar should go to Bede, an Anglo-Saxon monk who made the calculations way back in the 6th century. It was Pope Gregory XII who accepted Bede's work.1 (Oh, by the way, it's Saint Bede now.)
The Catholic countries Spain, Italy, Portugal, and France complied immediately, and several other Catholic states followed in the next few years.
The lost days are confusing enough; before the switch, the new year fell on March 25. To make the new calendar year work, England decreed that the new year started on January 1. As a result, 1752 turned into 1753 almost 3 months early.1752 was 72 days short of the usual year.
Even today this proves troublesome, since it is not always clear just what date you mean, a problem when studying history or geneaology. George Washington's mother would tell you her son was born on February 11, 1731 (and she has the stretch marks to prove it). Today's textbooks tell you the date was February 22, 1732. Both dates are correct, since they signify the same day historically.
You may occasionally see O.S. or N.S. next to an historical date--these mean Old or New Style, depending on whether the Julian or Gregorian calendars are being used.
Russia did not follow until 1918, jumping from January 31st right to February 14th. Given the brutality of the Russian winters, this makes sense. Unfortunately for the Russians, winter lasted the same as it always did. (Interestingly, the Russian calendar is even more aligned to the tropical year than the Gregorian--in a few thousand years, they'll add a leap year the Gregorian calendar does not recognize. Just one more thing for us to worry about.)
1 "XV. De mensibus Anglorum," De temporum ratione, A.D. 725. To be fair,though, I stole the reference from
Observing Bede's Anglo-Saxon Calendar, by John Robert Stone, 1997, http://www.kami.demon.co.uk/gesithas/calendar/obs_bede.html#bede
"Change of calendars - Sweden" http://www.algonet.se/~hogman/tiderakning_eng.htm
"The British Switch to the Gregorian Calendar" http://www.crowl.org/Lawrence/time/britgreg.html
in10se has a lovely node October 15, 1582 linked above that elegantly gives you the history of the original calendar change, more than a hundred years earlier.