http://www.adsb.co.uk/date_and_time/calendar_reform_1752/
Calendar Reform in England, 1752
It is widely known that in September 1752, Great Britain switched from the Julian Calendar to the Gregorian Calendar. In order to achieve the change, 11 days were ‘omitted’ from the calendar - i.e. the day after 2 September 1752 was 14 September 1752.
This change was as a result of an Act of Parliament - the “Calendar Act” of 1751 An Act for Regulating the Commencement of the Year; and for Correcting the Calendar now in Use.
What isn’t so widely known is a second change which the Act introduced - as named in the first part of the Act’s title. The Act changed the first day of the year (or, if you want to impress your friends with a new word, the Supputation of the Year).
Prior to 1752 in England, the year began on 25 March (Lady Day). Lady Day is one of the Quarter Days, which are still used in legal circles. The Quarter Days divide the year in quarters (hence the name :-), and the Quarter Days are: Lady Day (25 March), Midsummers Day (24 June), Michaelmas Day (29 September), and Christmas Day (25 December).
So, in England, the day after 24 March 1642 was 25 March 1643. The Act changed this, so that the day after 31 December 1751 was 1 January 1752. As a consequence, 1751 was a short year - it ran only from 25 March to 31 December.
To throw some more confusion on the issue, Scotland had changed the first day of the year to 1 January in 1600 (in 1600, Scotland was a separate kingdom). When King James VI of Scotland became also King James I of England in 1603, the possibilities of date confusion must have been very large.
Historians have to be on their toes with dates prior to 1752. For example, in The Tower of London there is some graffiti scratched into a cell wall by someone imprisoned in January 1642 for his role in the Battle of Edgehill (which took place on 23 October 1642).
And we were worried about Y2K?