Combine zucchini, onion,& salt in large bowl. Cover and refrigerate overnigh or 8 hours.
Rinse Zucchini mixture in colander. Drain and squeeze dry. Pat with paper toweling to dry further.
Place mixture, sugar, vinegar, celery seed, turmeric, and black pepper in large nonaluminum dutch oven. Simmer 30 minutes, stirring.
Meanwhile prepare your jars, lids and screw bands according to manufacturer's directions.
Ladle hot relish through a 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.