How to send mail based on date field?
To achieve this you have to use Rules as well as Scheduler module. Please follow the documentation links: Rules documentation: https://www.drupal.org/documentation/modules/rules Scheduler documentation: https://www.drupal.org/node/108984

You can also refer: http://www.webwash.net/tutorials/how-send-follow-emails-using-rules-scheduler-drupal-7