Combine mayonnaise, sugar, vinegar and mustard in bowl and mix thoroughly until sugar is completely dissolved. Using a large and very sharp chef's knife, slice cabbage into shreds fine shreds. Thinly slice or shred carrot and combine with cabbage in bowl. Sprinkle lightly with celery salt and toss. While stirring, pour just enough dressing over the prepared vegetables to moisten them nicely. (After it sits a while, there will suddenly be more liquid in the bowl than you thought there was- it will come off the veggies. So don't overdo the dressing. You can always add more later if need be.) Cover and chill. Refrigerate unused portion of dressing for later use.