code: public static HolidayCalendar<Calendar> toHolidayCalendarSet(final HolidayCalendar<Date> dates) {
          final Set<Calendar> calendars = new HashSet<>();
          for (final Date date : dates.getHolidays()) {
              calendars.add(getCal(date));
          }
          return new DefaultHolidayCalendar<>(calendars, getCal(dates.getEarlyBoundary()), getCal(dates.getLateBoundary()));
      }
docs: Converts a Set of Date objects to a Set of Calendar objects.
  
  @param dates
  @return the converted Set&lt;Calendar&gt;