%g/^BEGIN:VEVENT/,/^END:VEVENT$/ join
%!grep ^BEGIN:VEVENT

  # get 1st class feasts only:
%!grep ';I. cl.'

  # get the start dates:
%!grep -o 'DTSTART;VALUE=DATE:[0-9]\+'

  # numerical dates only:
%!grep -o "[0-9]\+"

  # sort unique:
%!sort -nu