The day before serving, scrub the potatoes and boil them in salted water until tender but still slightly firm they should not be mushy. Plunge them into ice water, and when cool, peel them and cut them into 3/4 inch dice. Season the potatoes with salt, pepper, and celery seed. Marinate the onion in the pickle for a few hours or overnight in the refrigerator. To serve, drain the juice and add the onions to the potatoes. Add the barbecue sauce and salad dressing, tossing to coat evenly. Place back in the refrigerator for at least one hour before serving, and just before serving, add the mayonnaise. Adjust the seasoning and serve.