public function onDaysOfTheWeek(array $onDaysOfWeek)
    {
        if(false == $onDaysOfWeek) {
            throw new \InvalidArgumentException('Days of week must be an non-empty set.');
        }

        foreach ($onDaysOfWeek as $day) {
            DateBuilder::validateDayOfWeek($day);
        }

        $this->daysOfWeek = $onDaysOfWeek;

        return $this;
    }