Sending emails on a specified interval, based on a date field
I've built a similar feature for the Commerce Recurring module, but instead of sending mails, it creates orders. Here's how I did it.

Interval field module would become handy so you can control how often the action happens (sending mails i.e.).

You could create your own cron event or just rely in the Drupal one, here's a the rule used for generating orders, yours would probably be simpler:

  $rule = rules_reaction_rule();
  $rule-&gt;label = t('Generate recurring orders on cron run');
  $rule-&gt;tags = array('Commerce recurring');
  $rule-&gt;active = TRUE;
  $rule
    -&gt;event('commerce_recurring_cron')
    -&gt;action('commerce_recurring_get_due_items', array(
      'number_items' =&gt; 5,
      'timestamp:select' =&gt; 'site:current-date',
      'provides' =&gt; array('commerce_recurring_entities', 'commerce_recurring_entity'),
    ));

  $actions = array(
    'commerce_recurring:select' =&gt; 'current-recurring-entity',
    'timestamp:select' =&gt; 'site:current-date',
  );

  $loop = rules_loop(array(
    'list:select' =&gt; 'commerce-recurring-entities',
    'item:var' =&gt; 'current_recurring_entity',
    'item:label' =&gt; t('Current recurring entity'),
  ))-&gt;action('commerce_recurring_provide_order_properties', array('commerce_recurring:select' =&gt; 'current-recurring-entity'))
  -&gt;action('commerce_recurring_generate_order_from_recurring', $actions);
  $rule-&gt;action($loop);

  $rules['commerce_recurring_cron_generate_orders'] = $rule;


The calculation of the due entities (commerce_recurring_get_due_items) is a rules action that calls this function, based in Entity Field Query to calculate those entities "due" to actions:

/**
 * Return recurring entities with due dates.
 */
function commerce_recurring_rules_get_due_items($number_items = 0, $due_date = NULL) {
  if (empty($due_date)) {
    $due_date = new DateObject();
    $due_date = $due_date-&gt;getTimestamp();
  }

  $query = new EntityFieldQuery();
  $query-&gt;entityCondition('entity_type', 'commerce_recurring', '=');
  $query-&gt;propertyCondition('status', TRUE, '=');
  $query-&gt;propertyCondition('due_date', $due_date, '&lt;');
  if ($number_items &gt; 0) {
    $query-&gt;range(0, $number_items);
  }
  $result = $query-&gt;execute();

  $recurring_entities = array();
  if (!empty($result['commerce_recurring'])) {
    foreach($result['commerce_recurring'] as $recurring_entity) {
      $recurring_entities[] = entity_load_single('commerce_recurring', $recurring_entity-&gt;id);
    }
  }

  return array('commerce_recurring_entities' =&gt; $recurring_entities);
}