Place the diced potatoes in a large pot and cover with cold water. Bring to a boil over medium-high heat and cook until tender, about 10-15 minutes. Drain and let cool.

In a large mixing bowl, combine the mayonnaise, Dijon mustard, apple cider vinegar, salt, and black pepper. Mix until smooth.

Once the potatoes are cool, add them to the bowl with the dressing. Gently fold in the celery, red onion, chopped hard-boiled eggs, and sweet pickle relish. If using, add the fresh dill.

Mix until all ingredients are well combined. Taste and adjust seasoning if necessary.

Cover and refrigerate for at least 1 hour before serving to allow flavors to meld.