Actually Mage_Catalog_Model_Category does possess the event _beforeSave(). In order to see that, you need to look on that class with wide eyes :)

Mage_Catalog_Model_Category directly does not define _beforeSave. However you can see that, this class inherits from another class Mage_Catalog_Model_Abstract and it does possess _beforeSave. It looks like this

 protected function _beforeSave()
{
    $this-&gt;unlockAttributes();
    return parent::_beforeSave();
}


You can see that, this class again call its parent class. That parent class is nothing but Mage_Core_Model_Abstract. There you can see that _beforeSave defined like this.

protected function _beforeSave()
{
    if (!$this-&gt;getId()) {
        $this-&gt;isObjectNew(true);
    }
    Mage::dispatchEvent('model_save_before', array('object'=&gt;$this));
    Mage::dispatchEvent($this-&gt;_eventPrefix.'_save_before', $this-&gt;_getEventData());
    return $this;
}


This function dispatches two events. Second one is important here. You can see that, it uses an event prefix in that event. Now take a look on Mage_Catalog_Model_Category. You can see that, it actually defines an event prefix there.

protected $_eventPrefix     = 'catalog_category';


This means, whenever category section is trying to save, _beforeSave method generates a unique event with name catalog_category_save_before. I think , you can use that event to save any category attribute

Hope that helps

Validate Category Attribute
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