How to create a rule triggered when saving a node to schedule sending an email at specified date?
Part 1 - Make sure to include "Entity has field"

First eliminate the possibility that the missing piece to make your rule work is because you didn't include a Rules Condition like "Entity has field" (related to your Reminder date field), as the first Rules Condition.

If you didn't do that yet, then to Rules it is like the value of your Reminder date is simply blank. If needed, enable Rules debugging to double check if the value of that Reminder data has indeed the value that you expect. Refer to my answer to "How can I display the value of a variable inside a condition?" for more details about Rules debugging.

Part 2 - Rework your rule to use the Rules Scheduler

This seems to be a perfect case to actually use the Rules Scheduler (sub-module of Rules). Have a look at my answer to "Rule to store user&#39;s last post date and scheduled rule based 30 day reminder", which contains an example of how to use the Rules Scheduler for a variation of your question. The variation is that it is to only eMail 1 user instead of a list of users as in your case, though it uses a field that is similar to your Reminder date field.

The approach using the Rules Scheduler differs from what you tried to do (but couldn't get to work):


it does NOT use a Rules Condition like "check if this the right moment to send such eMail (now)".
instead it performs appropriate Rules Action to schedule such eMails for somewhere in the future. Moreover, it takes into account that the actual date for that eMail may even change in between (ie if the Reminder date field gets updated, then don't send that previously requested eMail anymore).