In a large nonmetal bowl combine squash, onion, and sweet pepper. Sprinkle salt over vegetables; stir to combine. Cover and refrigerate for 1 hour. Drain off liquid.
In a 3-quart saucepan combine sugar, vinegar, mustard seed, celery seed, 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 (sixteen 1/4-cup servings).