code: public DayOfTheWeek next() {
          if (this == PH) {
              return null;
          }
          final int nextId = id + 1;
          if (nextId == PH.id) {
              return MON;
          }
          for (final DayOfTheWeek dow : ALL) {
              if (dow.id == nextId) {
                  return dow;
              }
          }
          throw new IllegalStateException("Wasn't able to find next day for: " + this);
      }
docs: Returns the next day.
  
  @return Day that follows this one. In case of {@link #PH} {@literal null} is returned.