Here's an example cron. It runs at 3am/3pm daily. Only if the website is not undergoing maintenance.
0 3,15 * * * ! test -e /var/www/html/maintenance.flag && /usr/bin/php /var/www/html/shell/activeCategories.php
You could use something like this.
0 3,15 * * * /usr/bin/php /var/www/html/shell/activeCategories.php