You could also look at Mailchimp.com and the automation it offers. You can easily create a database of your customers including custom fields for specific service dates, then it automates the sending of a specific emails when the service date/s are reached.

I have set this up previously for an airconditioning maintenance company that also had yearly maintenance services, Mailchimp worked really well. The reports also let you know who has read the emails and which links within the email the customer clicked on.

If you have any further questions then let us know.