A turkey (and the cook!) should rest about 30 minutes after baking to let the juices reabsorb, so pop the casserole in as soon as you remember it. Most cook quickly, so it shouldn't take too long. If your guests are anxious to get eating, go ahead and carve the bird and bring the dish out when it's done. By the time everyone has started serving, your forgotten dish will be piping hot and ready to eat.