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 30 minutes before serving.