As Braunson has suggested, setting up a script to do this would definitely be the preferred option. You only need to schedule the script to run once a day, and if it's likely to be processor-intensive then there's no reason why you can't run it first thing in the morning or later on in the evening. What you might want to do is add a column (flag) in your table schema so that the client's record is updated as soon as the notification has been sent.
The important thing to make sure first is that your hosting provider allows you to schedule cron jobs in some way. If this is ok then you just need to write a database query to identify all of the clients that need to be notified, and then put it in a PHP script along with a basic form mailer routine or library. I would be tempted to probably send another notification 10 working days prior to the event, because it's easy to lose track of things after a month (well, it is for me anyway).