Until the time of Julius Caesar the Roman year was organized round the phases of the moon. For many reasons this was hopelessly inaccurate so, on the advice of his astronomers, Julius instituted a calendar centered round the sun. It was decreed that one year was to consist of three hundred and sixty-five and a quarter days, divided into twelve months; the month of Quirinus was renamed 'July' to commemorate the Julian reform. Unfortunately, despite the introduction of leap years, the Julian calendar overestimated the length of the year by eleven minutes fifteen seconds, which comes to one day every on hundred and twenty-eight years. By the sixteenth century the calendar was ten days out. In 1582 reforms instituted by Pope Gregory XIII lopped the eleven minutes fifteen seconds off the length of a year and deleted the spare ten days. This new Gregorian calendar was adopted throughout Catholic Europe.
Protestant Europe was not going to be told what day it was by the Pope, so it kept to the old Julian calendar. This meant that London was a full ten days ahead of Paris. The English also kept the 25th of March as New Year's Day rather than the 1st of January. By the time England came round to adopting the Gregorian calendar, in the middle of the eighteenth century, England was eleven days ahead of the Continent.
A Calendar Act was passed in 1751 which stated that in order to bring England into line, the day following the 2nd of September 1752 was to be called the 14th, rather than the 3rd of September. Unfortunately, many people were not able to understand this simple manoeuvre and thought that the government had stolen eleven days of their lives. In some parts there were riots and shouts of 'give us back our eleven days!'
Before the calendar was reformed, England celebrated Christmas on the equivalent of the 6th of January by our modern, Gregorian reckoning. That is why in some parts of Great Britain people still call the 6th of January, Old Christmas Day.