In a 3-quart saucepan combine sugar, vinegar powder, mustard seed, celery seed, water, and ground mustard. Heat to boiling. Add squash mixture. Return to boiling; remove from heat. Ladle mixture into 2 clean pint jars. Cool 30 minutes. Cover and refrigerate at least 24 hours before serving. Store in refrigerator up to 1 month. Makes 2 pints. (16 1/4 cup servings)