Old Style- New Style
Old Style means computed according to the unreformed calendar. New Style means computed according to the calendar reformed and corrected by Gregory XIII. in 1582. The New Style was introduced into England, in 1752, during the reign of George II., when Wednesday, September 2nd, was followed by Thursday, September 14th. This has given rise to a double computation, as Lady Day, March 25th, Old Lady Day, April 6th; Midsummer Day, June 24th, Old Midsummer Day, July 6th; Michaelmas Day, September 29th, Old Michaelmas Day, October 11th; Christmas Day, December 25th, Old Christmas Day, January 6th.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894