int days = (datePicker1.SelectedDate.Value.Day-DateTime.DaysInMonth(datePicker1.SelectedDate.Value.Year, datePicker1.SelectedDate.Value.Month));
            int wholeWeeks = days / 7;
            int remainder = days % 7;
            
            string result = days + (Math.Abs(days) == 1 ? " day" : " days");
            
            if (Math.Abs(days) > 13)
                result += wholeWeeks.ToString();
            else
                result += wholeWeeks.ToString();

1. 1-4
2. 5-11
3. 12-18
4. 19-25
5. 26-31

procedure TForm1.Button2Click(Sender: TObject);
var
  today: TDateTime;
  year, month, day: word;
  dow, max, line, no: integer;
  sl: TStringList;
  s: string;
begin;
  today:=Date();
  DecodeDate(today, year, month, day);
  dow:=(Trunc(today-day+1)+5) mod 7;
  max:=DaysInAMonth(year, month);

  ListBox1.Items.Clear;
  for line:=0 to 6 do begin;
    no:=line+1-dow;
    s:='';
    repeat;
      if no<=0 then s:=s+'     ' else s:=s+Format('%.2d ',[no]);
      inc(no,7);
      until no>max;
    ListBox1.Items.Add(s);
    end;
  end;

today:=Date(); // нужная дата
  DecodeDate(today, year, month, day);// ????? 
  dow:=(Trunc(today-day+1)+5) mod 7;// ?????
  max:=DaysInAMonth(year, month);// Максимальное кол-во дней в месяце

private void button1_Click(object sender, RoutedEventArgs e)
        {
          //  ArrayList data = new ArrayList();
            for (line = 0; line <= 6; line++)
            {
                s_day = "";
                for (month_int = 2; month_int <= 12; month_int++)
                {
                    dow = (int)(new DateTime(2012, month_int, 1, 0, 0, 0)).DayOfWeek;
                    max = System.DateTime.DaysInMonth(2012, month_int);                 //Максимальное количество дней в текущем месяце
                    w = (int)(new DateTime(2012, month_int, 1, 0, 0, 0)).Month;
                    no = line + 1 - dow;
                    s = "";
                    do
                    {
                        if ((no + 7) >= max)
                        {
                            if (no <= 0) { s = s + "| "; }
                            else { s = s + "  " + no.ToString("00", CultureInfo.InvariantCulture); }
                        }
                        else
                        {
                            if (no <= 0) { s = s + "| "; }
                            else { s = s + "  " + no.ToString("00", CultureInfo.InvariantCulture); };
                        }
                        no = no + 7;
                    } while (no <= max);
                    s_day = s_day + s;
                 }

                listBox1.Items.Add(s_day);
               // data.Add(s_day);
            //    dataGrid1.ItemsSource=data;
            }
         //   return data;
        }