Как узнать номер недели данного дня в году?

Вариант 1:

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;

Вариант 2:

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;

Вариант 3:

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;

Взято из https://forum.sources.ru

Получить номер недели по дате

var
   FirstWeekDay: Integer = 2;
   { Wochentag, mit dem die Woche beginnt 
   (siehe Delphi-Wochentage) 
   2 : Montag (nach DIN 1355) }
   FirstWeekDate: Integer = 4;
   { 1 : Beginnt am ersten Januar 
    4 : Erste-4 Tage-Woche (nach DIN 1355) 
    7 : Erste volle Woche }
 
   { liefert das Datum des ersten Tages der Woche }
   { get date of first day of week}
 function WeekToDate(AWeek, AYear: Integer): TDateTime;
 begin
   Result := EncodeDate(AYear, 1, FirstWeekDate);
   Result := Result + (AWeek - 1) * 7 - ((DayOfWeek(Result) + (7 - FirstWeekDay)) mod 7);
 end;
 
 { liefert die Wochennummer und das Jahr, zu dem die Woche gehort }
 { get weeknumber and year of the given week number}
 procedure DateToWeek(ADate: TDateTime; var AWeek, AYear: Word);
 var
   Month, Day: Word;
 begin
   ADate := ADate - ((DayOfWeek(ADate) - FirstWeekDay + 7) mod 7) + 7 - FirstWeekDate;
   DecodeDate(ADate, AYear, Month, Day);
   AWeek := (Trunc(ADate - EncodeDate(AYear, 1, 1)) div 7) + 1;
 end;
 
 
 {Week to date example}
 procedure TForm1.Button1Click(Sender: TObject);
 begin
   ShowMessage(FormatDateTime('dd.mm.yyyy', WeekToDate(51, 2000)));
 end;
 
 {Date to week example}
 procedure TForm1.Button2Click(Sender: TObject);
 var
   week, year: Word;
 begin
   DateToWeek(now, week, year);
   ShowMessage(IntToStr(week));
   ShowMessage(IntToStr(year));
 end;

Взято с сайта: https://www.swissdelphicenter.ch