Подскажите в чем причина?
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String month = reader.readLine();
SimpleDateFormat dateFormat = new SimpleDateFormat("MMMMM", Locale.ENGLISH );
Calendar calendar = new GregorianCalendar();
calendar.setTime(dateFormat.parse(month));
int m = calendar.get(Calendar.MONTH) + 1;
System.out.println(month + " is the " + m + " month");