public function addDay($day)
    {
        $available = array('mon', 'tue', 'wed', 'thu', 'fri', 'sat', 'sun');

        foreach ((array) $day as $value) {
            // only accept valid values
            if ( ! in_array($value, $available)) {
                throw new InvalidArgumentException('Day must be a valid value: '.implode(', ', $available));
            }

            $this->days[] = $value;
        }

        return $this;
    }