Combine the brown sugar, salt, paprika, chili powder, black pepper, mustard, cayenne, and celery salt in a bowl.
Pork
Season the pork with the rub for 45 minutes before placing it on the smoker.
Prepare a smoker to 250 degrees F (121 degrees C).
Place the shoulder on the pit and smoke to an internal temperature of 170 degrees F (77 degrees C),. This takes about 4 hours.
Remove the shoulder and dust with rub once more. Wrap tightly in foil and put back on the pit. Cook until an internal temperature of 200 degrees F (94 degrees C). This takes about 5 hours.
Let rest in a preheated dry cooler for a couple of hours before serving.