code: public function isDateAvailable()
      {
          // @todo For now, any out of scope variable inside of "beforeShowDay" method will be undefined.
          // @link https://github.com/refactoror/SelBlocks/issues/5#issuecomment-68511965
          $beforeShowDay = $this->datePicker(['option', 'beforeShowDay']);
  
          if (!empty($beforeShowDay) && !empty($this->execute("$beforeShowDay($this->date)")[0])) {
              throw new \Exception(sprintf('The "%s" is not available for choosing.', $this->date));
          }
  
          return $this;
      }
docs: @throws \Exception
  
  @return self