To obviate confusion, it should be pointed out that the dates of fixed-date observances are the same in both old (Julian) and new (Gregorian) calendars, though the actual day is different, since the old calendar lags the new by 13 days.  Holy Great Pascha and all days occurring so many days before or after Pascha are the same actual day everywhere, though their dates in the two calendars will of course not be the  same.  But fixed dates of Saints' days and other occasions occur according to the different calendars.  So while the Lordsdays of St. Mary of Egypt and St. Gregory Palamas (falling on a given number of days before Pascha) and Ascension and Pentecost or Trinity Lordsday (falling on a given number of days after Pascha) are the same everywhere--because, as just said, Pascha is the same everywhere in Orthodoxy--some fixed-date Saint's days and even a fixed-date major festival can fall within the Great Fast or the season of Pentecost on one calendar but not on the other.  Two important festivals can thus conflict on one calendar but not on the other, with different results.  The "havoc" referred to in the link below refers to this problem with the new calendar.   

Click the following links for the topics indicated:

THE TRADITIONAL CALENDAR OF THE ORTHODOX CHURCH

LITURGICAL HAVOC CAUSED BY THE NEW CALENDAR