docs: private static boolean _isValidInYear (@Nonnull final Holiday aHoliday, final int nYear)
    {
      return (aHoliday.getValidFrom () == null || aHoliday.getValidFrom ().intValue () <= nYear) &&
             (aHoliday.getValidTo () == null || aHoliday.getValidTo ().intValue () >= nYear);
    }