Google Flights is good for this (flights.google.com). Select MSP-HRL one-way, non-stop flights, and Delta as carrier. You can then click in the date field and a calendar will appear and you can forward through the months to see when a price appears (which indicates a flight is operating that day). A quick search on Google Flights shows there is no restart date in the schedule for DL. Sun Country is currently flying this route less than daily (guessing this might be part of the reason why DL flew it).