In a medium bowl, whisk together 1 cup mayonnaise, 1 cup sour cream, and ¼ cup buttermilk until smooth.
Stir in 2 teaspoons garlic powder, 1 tablespoon onion powder, 1 tablespoon parsley, ½ teaspoon dill, ½ teaspoon salt, ½ teaspoon mustard powder, and ¼ teaspoon celery salt (if using). Mix well until everything is combined.
Cover and refrigerate for at least 30 minutes before serving to allow the flavors to meld. Enjoy!