Originally Posted by ijgordon

If it's a fully refundable fare, you can cancel at any time, even after the flight has been completed. (Technically you don't need to cancel the reservation, that will happen automatically when you no-show, so you can just request the ticket refund at any time afterward).
If it's non-refundable, I believe the fare rules technically require you to cancel the reservation prior to the departure of the flight in order to get a credit for the value, but I believe Delta is not enforcing that because of long hold times to speak to reservations (as not all itineraries can be canceled online).