You can do it via cron. In fact, you may even have something for this already. Check under /etc/cron.weekly and see if you have something about yum.
If you edit this file, it's pretty much doing a weekly update, but it requires that /var/lock/subsys/yum be present on your system. So just do a touch /var/lock/subsys/yum and updates will start happening weekly. You really should do a "man yum" to see what all of those options mean. For example, -y means to answer "yes" to any questions yum would normally ask you in interactive mode. You should be sure that you're happy with answering "yes" to all questions. Usually it's safe to say "yes" but you may not like doing that.