In a bowl, combine the mayonnaise, mustard, sugar, vinegar, buttermilk, celery seeds, salt and pepper; whisk well to dissolve the sugar.
In a large bowl, combine the remaining ingredients. Toss with the dressing until evenly coated. Adjust seasoning, to taste. Place in the refrigerator, covered, to chill and meld flavors for at least 1 hour before serving.