docs: protected final void addChrstianHoliday (final ChronoLocalDate aDate,
                                             final String sPropertiesKey,
                                             final IHolidayType aHolidayType,
                                             final HolidayMap holidays)
    {
      final LocalDate convertedDate = LocalDate.from (aDate);
      holidays.add (convertedDate, new ResourceBundleHoliday (aHolidayType, sPropertiesKey));
    }