The Greek Orthodox Church has adopted the Gregorian Calendar in 1924, 
while civil authorities converted in 1923.  Hence in Greece we 
celebrate Christmas on the 25th of December.  There is, however, a 
separate church of the so-called Genuine Orthodox Christians or 
Old-Calendarists who still use the Julian Calendar.  The Julian 
Calendar is used, however, by the Church of Greece for the 
calculation of the Easter Day and in Mount Athos where almost all 
monasteries are not "Genuine Orthodox Christians" the Julian calendar 
is still in use.

BTW, the algorithm for converting Julian to Gregorian dates is more 
complicated than subtracting or adding 13 or 11 days.  For example, 
in Smith's time the difference was 11 days wheras in Keynes' time the 
difference was 12 days.  "The Great October Socialist Revolution" was 
of course celebrated, as Barkley Rosser points out, on Nov 7.  (13 
days difference).  A very useful site for this conversion is that of 
the Institut de mecanique celeste et de calcul des ephemerides (in english).
http://www.imcce.fr/page.php?nav=en/ephemerides/astronomie/calendriers/correspond/cal429.php

Nicholas J. Theocarakis