Keep refrigerated in a tightly capped container and the dressing will last 3 weeks without spoilage. –The White House, Wilmington, Vermont
Put buttermilk in saucepan and heat until it simmers. Let cool. Place mayonnaise, mustard, garlic, celery seed, and salt and pepper in large bowl. Whisk together. When buttermilk is cool, add to mayonnaise mixture. Pour mixture into tightly capped container and refrigerate.