Zen Cart by default requests a customer to enter his/her date of birth during registration. This is optional of course and can be removed from the Zen Cart admin panel. However one can make good use of this to regularly wish their customers a happy birthday thereby keeping in touch with them.
I have developed a script which will send a birthday wish to the customer when their birth date comes along. You will need to setup a cron job on your hosting server to run the script each day. You are also free to make changes to the script according to your needs.
Here is the Zen Cart birthday script and hope it helps some one.
Remember to copy it to your root folder and then setup a cron job to execute it daily.