<config>
...
    <crontab>
        <jobs>
            <catalogrule_apply_all>
                <schedule><cron_expr>0 1 * * *</cron_expr></schedule>
                <run><model>catalogrule/observer::dailyCatalogUpdate</model></run>
            </catalogrule_apply_all>
        </jobs>
...
    </crontab>
...
</config>

http://www.magentocommerce.com/wiki/1_-_installation_and_configuration/how_to_setup_a_cron_job

Magento allows you to schedule custom tasks in an XML configuration, in a similar manner to the UNIX crontab style. Here is an example from app/code/core/Mage/CatalogRule/etc/config.xml:

Magento How to Set Up a Cron Job

PHP