Combine cabbage, onion, green pepper, carrot, celery seeds, salt and pepper in a large bowl. Set aside. Combine sugar, vinegar, oil and dry mustard in a saucepan and bring to a boil on top of the stove. Remove and let cool. Toss dressing with slaw mixture. Refrigerate several hours or overnight. Makes 7 to 8 cups coleslaw. Keeps up to a week refrigerated.