Here is a current Northwest promotion. You can book your tickets on www.nwa.com. This is what their email has to say:
Book your summer vacation today with the Northwest Airlines Summer Sale. For a limited time enjoy extra savings when purchasing tickets for travel between the South and select U.S. cities. To take advantage of airfares as low as $84*, purchase your tickets at least 14 days prior to departure but no later than May 18, 2009 and complete your travel by September 14, 2009. Fares shown are valid on Tuesday and Wednesday, are one way and require a roundtrip purchase. Seats are limited and blackout dates apply, so purchase today!
Purchase on nwa.com before May 31, 2009 and receive 500 WorldPerks Bonus Miles.
*Fares shown are each-way per person in USD for travel in Coach Class, require a roundtrip purchase and are available at nwa.com. Additional taxes/fees/restrictions/baggage charges may apply.
Typically these types of sale only apply to certain originations and destinations. If it happens to include where you are travelling that is great!