Occasionally airbnb will prompt you to make a special offer, say at 10% off. This is the only way I can see to make a special offer unless you already are in communication with a renter. However, you can always tamp the price down for a certain period or particular days or weeks and indicate you have a special on for those days in the opening description in your listing. If anyone has a way to actually "post a special offer" on your own for a certain week or weeks or days, please let me know; I would be most interested in doing this as unreserved days come closer!