So if read ur last post right u need 3 classes:
class MonthNumberException extends Exception {}
class Month {}
class MonthDemo {}

the way use for instans Month methodes in MonthDemo is that u need to make an object of it in MonthDemo. e.g.
class Food {
String eatMe() {.....};

class Mouth {
Food tmp;

tmp = new Food();//make a usable object og it somwhere in ur code, constructor, main methode, other methodes u will call
tmp.eatMe(); // use the methode from object tmp that is of the food type.

If don't jet understand basic oop i would recomend some of sun tutorials:

Lesson: Classes and Objects (The Java™ Tutorials > Learning the Java Language) and Using Objects (The Java™ Tutorials > Learning the Java Language > Classes and Objects)