code: @Nullable
      public static DayOfTheWeek valueOf(@Nullable final String str) {
          if (str == null) {
              return null;
          }
          for (final DayOfTheWeek dow : ALL) {
              if (dow.value.equalsIgnoreCase(str)) {
                  return dow;
              }
          }
          throw new IllegalArgumentException("Unknown day of week: '" + str + "'");
      }
docs: Converts a given string into an instance of this class.
  
  @param str
  String to convert.
  
  @return New instance.