In a large mixing bowl, combine cabbage, onion, carrots, and celery. Sprinkle with the sugar, and toss well. In a small saucepan, combine vinegar, oil, salt, dry mustard, and pepper. Bring to a boil. Pour hot dressing over cabbage mixture, and mix well. You can eat it in as little as 30 minutes, but it will keep in your refrigerator for 2 weeks! Some say it gets better each day.