http://www.delta.com/content/www/en_...cellation.html
Risk-free Cancellation
Tickets/Cancellation:
Cancellation request must be made by midnight of the day after the eTicket is purchased or midnight of the departure date of the first flight, whichever comes first. Available only for eTickets purchased at the time of reservation through Delta ticket offices and airport ticket counters, Reservation Sales, or at delta.com.