Place a large pot of salted water over medium high heat and bring to a boil.
Add potatoes. Cook until tender but still firm, about 15 minutes.
Drain in a colander. Allow to cool.
When cool to the touch, chop into 1/2 to 3/4-inch cubes.
Place potatoes in a large mixing bowl.
Add mayonnaise, mustard, celery, dill pickles or relish, onion and pimento. Mix gently, but well.
Taste. Add salt and pepper, to taste.
Store in a tightly covered container, in the refrigerator, until cooled or ready to serve, up to 3 days.