code: public static boolean isValid(@Nullable final String dayOfTheWeek) {
          if (dayOfTheWeek == null) {
              return true;
          }
          for (final DayOfTheWeek dow : ALL) {
              if (dow.value.equalsIgnoreCase(dayOfTheWeek)) {
                  return true;
              }
          }
          return false;
      }
docs: Verifies if the string is a valid dayOfTheWeek.
  
  @param dayOfTheWeek
  Weekday string to test.
  
  @return {@literal true} if the string is a valid number, else {@literal false}.