function GetCalendarInfo(Locale: LCID; Calendar: DWORD; CalType: DWORD; lpCalData: LPTSTR;
  cchData: Integer; lpValue: LPDWORD): Integer; stdcall; external kernel32 name 'GetCalendarInfoW';

const
  LOCALE_RETURN_GENITIVE_NAMES = $10000000;   //Flag to return the Genitive forms of month names
  LOCALE_ALLOW_NEUTRAL_NAMES   = $08000000;   //Flag to allow returning neutral names/lcids for name conversion

  CAL_GREGORIAN                = 1;      // Gregorian (localized) calendar

  CAL_SMONTHNAME1           = $00000015;  // native name for January
  CAL_SMONTHNAME2           = $00000016;  // native name for February
  CAL_SMONTHNAME3           = $00000017;  // native name for March
  CAL_SMONTHNAME4           = $00000018;  // native name for April
  CAL_SMONTHNAME5           = $00000019;  // native name for May
  CAL_SMONTHNAME6           = $0000001a;  // native name for June
  CAL_SMONTHNAME7           = $0000001b;  // native name for July
  CAL_SMONTHNAME8           = $0000001c;  // native name for August
  CAL_SMONTHNAME9           = $0000001d;  // native name for September
  CAL_SMONTHNAME10          = $0000001e;  // native name for October
  CAL_SMONTHNAME11          = $0000001f;  // native name for November
  CAL_SMONTHNAME12          = $00000020;  // native name for December
  CAL_SMONTHNAME13          = $00000021;  // native name for 13th month (if any)

// AIndex is 1-based
function GetMonthName(AIndex: Integer): UnicodeString;
var
  CalType: DWORD;
  L: Integer;
begin
  Result := '';
  CalType := (CAL_SMONTHNAME1 + AIndex - 1) or LOCALE_RETURN_GENITIVE_NAMES;
  L := GetCalendarInfo(LOCALE_USER_DEFAULT, CAL_GREGORIAN, CalType, nil, 0, nil);
  if L < 2 then Exit;
  SetLength(Result, L - 1);
  GetCalendarInfo(LOCALE_USER_DEFAULT, CAL_GREGORIAN, CalType, PWideChar(Result), L, nil);
end;