Doesn't your cert signing CA send a notification ? Public CA like Entrust does it routienly.
How many certs are you dealing with? I have worked in a large enviroment where a script would fetch all the certs across multilple LBs and someone would create a tracking report based on that and do a TODO assignment list for a team of people to renew it before expiry. Yould could probably do a cron job but it will be taxing on CPU and from past experience security team doesn't like the email sending idea. Your shop might be different.