DouKaku?

#!/usr/bin/env ruby
require 'date'

from = DateTime.now
to = DateTime.parse("2013-12-31")

friday = (from..to).inject(0) do |friday, date|
    if date.mday == 13 and date.wday == 5 then
        puts date.strftime('%Y-%m-%d')
        friday + 1
    else
        friday
    end
end

puts friday

program Doukaku197;

{$APPTYPE CONSOLE}

uses
  SysUtils;

var
  Day, Max: TDateTime;
  Y, M, D:  Word;
  I:        Integer;
begin
  Day := Date;
  Max := EncodeDate(2013, 12, 31);
  I   := 0;
  while Day <= Max do
  begin
    DecodeDate(Day, Y, M, D);
    if (D = 13) and (DayOfWeek(Day) = 6) then
    begin
      WriteLn(Y, '/', M, '/', D);
      Inc(I);
    end;
    Day := Day + 1;
  end;
  WriteLn(I, ' days');
end.