No se que pasa, Funciona correctamente con todos los meses.
package com.codegym.task.task08.task0828;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Set;
/*
Month number
*/
public class Solution
{
public static void main(String[] args) throws IOException
{
//write your code here
HashMap<String, Integer> meses = new HashMap<String, Integer>();
meses.put("January", 1);
meses.put("February", 2);
meses.put("March", 3);
meses.put("April", 4);
meses.put("May", 5);
meses.put("June", 6);
meses.put("July", 7);
meses.put("August", 8);
meses.put("September", 9);
meses.put("Octuber", 10);
meses.put("November", 11);
meses.put("December", 12);
BufferedReader bufer = new BufferedReader(new InputStreamReader(System.in));
String mes = bufer.readLine();
int num = meses.get(mes);
System.out.println("\"" + mes + " is month " + num + "\"");
}
}