Since 1924 the Church of Greece has followed the new (Revised Julian) calendar for fixed feasts, which currently coincides with the civil Gregorian calendar. Christmas is therefore celebrated on 25 December.
Some Orthodox Churches — such as those of Russia, Serbia and Jerusalem — Mount Athos, and the Old Calendarists in Greece follow the Julian (old) calendar, which is currently 13 days behind. They therefore keep the saints' days 13 days later on the civil calendar: Christmas falls on 7 January and the feast of Saint Demetrius on 8 November.
Every date page also gives the corresponding day for those who follow the old calendar.
What about Easter?
Almost all Orthodox Churches, whether on the new or the old calendar, calculate Easter the same way (a well-known exception is the Orthodox Church of Finland). Movable name days such as Lambros or Zoe therefore fall on the same day on both calendars. See the guide to movable name days.
The 13-day difference applies to dates from 1900 until February 2100; from March 2100 it becomes 14 days.