...
uses DateUtils;
function DayName(aDate: TDate): string;
begin
case GetUserDefaultLangID and $03FF of
LANG_SPANISH: Result := LongDayNames[DayOfTheWeek(aDate)+1];
LANG_ENGLISH: Result := LongDayNames[DayOfTheWeek(aDate)];
end;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
ShowMessage(DayName(StrToDate('22/08/2013')));
end;