In a medium bowl combine the tomatoes, zucchini, and celery; set aside.
For dressing, in a screw-top jar combine the lemon juice, sugar, onion, vinegar, olive oil, salt, mustard, and poppy seeds. Cover; shake until well combined. Pour dressing over vegetable mixture. Cover; chill 4 to 24 hours.
Serve with a slotted spoon as a relish for bratwurst, burgers, or ham. Makes 2-1/2 cups (ten 1/4-cup servings).