This is a highly debated subject, as the first week of the year may be considered to be before or after Jan. The following function provides a starting point for WeekOfYear calculations. Example:

function WeekOfYear(ADate : TDateTime) : word;
var
  day : word;
  month : word;
  year : word;
  FirstOfYear : TDateTime;
begin
  DecodeDate(ADate, year, month, day);
  FirstOfYear := EncodeDate(year,  ,  );
  Result := Trunc(ADate - FirstOfYear) div 7 +  ;
end;
procedure TForm .Button Click(Sender: TObject);
begin
  ShowMessage(IntToStr(WeekOfYear(Date)));
end;