Maybe you can add an example database here!
For me is not clear, what you really have, and what you want to do.
Code:
It is an events database where I want the user to start entering the details of a new event on the main form...
Your main form is events form? And all those events have some dates?
And you have some subform in events form. But on which data is this subform based, and what it does?
Code:
When the user enters a new event on 04/10/19, I want the subform to update and show 3 dates already registered on the same day.
Deeper into forest there is more trees! Does this subform also display same events, or what?