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;

function WeekNum(const ADate: TDateTime): word;
var
 Year: word;
 Month: word;
 Day: word;
begin
 DecodeDate(ADate + 4 - DayOfWeek(ADate + 6), Year, Month, Day);
 result := 1 + trunc((ADate - EncodeDate(Year, 1, 5) +
  DayOfWeek(EncodeDate(Year, 1, 3))) / 7);
end;

function WeekOfYear(Dat: TDateTime): Word;
// Интерпретация номеров дней:
// ISO: 1 = Понедельник, 7 = Воскресенье
// Delphi SysUtils: 1 = Воскресенье, 7 = Суббота
var
 Day, Month, Year: Word;
 FirstDate: TDateTime;
 DateDiff: Integer;
begin
 day := SysUtils.DayOfWeek(Dat) - 1;
 Dat := Dat + 3 - ((6 + day) mod 7);
 DecodeDate(Dat, Year, Month, Day);
 FirstDate := EncodeDate(Year, 1, 1);
 DateDiff := Trunc(Dat - FirstDate);
 Result := 1 + (DateDiff div 7);
end;