Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and let cool.
Mix mayonnaise, buttermilk, Dijon mustard, dill, red onion, and celery in a large salad bowl; gently fold in the potatoes. Season with salt and black pepper. Chill for 4 hours before serving.
