We find ourselves wanting to send messages based not on dates relative to when a communication plan is applied, and not on specific dates, but on dates relative to those that appear on a person's record. An easy example is a birthday, but there are others (say, "7 days after a person is accepted," or "12 days after a student is enrolled").
This request is similar to the automatic birthday email message request (https://support.populiweb.com/hc/en-us/community/posts/41311058721947-Automation-Birthday), albeit more communication plan based.
Date fields like birthday, term start date, various application processing dates (as suggested in https://support.populiweb.com/hc/en-us/community/posts/214253067-Event-based-trigger-in-communication-plan), etc., would be handy. Perhaps as a link in the date selection area that would display available date fields to be used.