function WeekOfYear(ADate: TDateTime): word;
var
  day: word;
  month: word;
  year: word;
  FirstOfYear: TDateTime;
begin
  DecodeDate(ADate, year, month, day);
  FirstOfYear := EncodeDate(year, 1, 1);
  Result := Trunc(ADate - FirstOfYear) div 7 + 1;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(IntToStr(WeekOfYear(Date)));
end;