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