code: public DayOfTheWeek previous() {
          if (this == PH) {
              return null;
          }
          final int nextId = id - 1;
          for (final DayOfTheWeek dow : ALL) {
              if (dow.id == nextId) {
                  return dow;
              }
          }
          return SUN;
      }
docs: Returns the previous day.
  
  @return Day before this one. In case of {@link #PH} {@literal null} is returned.