code: @Override
      protected PlainDate getDate0(int year) {
  
          int month = this.getMonthValue();
          int lastDay = GregorianMath.getLengthOfMonth(year, month);
          int lastW = GregorianMath.getDayOfWeek(year, month, lastDay);
          int delta = (lastW - this.dayOfWeek);
  
          if (delta < 0) {
              delta += 7;
          }
  
          return PlainDate.of(year, month, lastDay - delta);
  
      }
docs: ~ Methoden ----------------------------------------------------------