Combine zucchini, onion and salt in large bowl. Cover and refrigerate 8 hours or overnight.
Rinse zucchini mixture in colander. Drain; squeeze dry. Pat with paper toweling to dry further.
Place zucchini mixture, sugar, vinegar, celery seed, turmeric and pepper in large nonaluminum Dutch oven. Simmer 30 minutes, stirring.
Meanwhile, sterilize canning jars, lids and screw bands according to manufacturers directions.
Ladle hot relish through funnel into hot jars, leaving 1/2-inch headspace. Cover with lids; screw on bands. Process jars in large pot simmering water for 15 minutes; jars should be covered by 1 inch of water. Remove jars to rack to cool. Test seals. Store in cool, dark place at least 2 weeks before using. Makes 7 half-pints.