private String fullMonth(Integer aMonth){
    String result = "";
    if(aMonth != null){
      if(fCustomLocalization != null){
        result = lookupCustomMonthFor(aMonth);
      }
      else if (fLocale != null){
        result = lookupMonthFor(aMonth);
      }
      else {
        throw new IllegalArgumentException("Your date pattern requires either a Locale, or your own custom localizations for text:" + Util.quote(fFormat)) ;
      }
    }
    return result;
  }