Mix the mayonaise, mustard, and celery seed in a large serving bowl.
Add cubed potatoes, relish and chopped celery and mix lightly. Season with salt and pepper. Refrigerate.
Cooks tip: You can eat this potato salad right away, but it is better to let it sit for the flavors to blend. I make the potatoes the day before and chill them. You can use other potatoes, but red potatoes absorb dressing readily.