Combine the cucumbers, sweet onion, green and red peppers, and salt in a large bowl. Toss to combine, cover with a kitchen towel, and let stand in a cool area overnight or for at least four hours.

Drain and rinse the vegetables in a colander. Rinse several times, pressing the vegetables with the back of a wooden spoon to remove all liquid and salty residue. Set aside.

Sterilize 6 half-pint jars, lids, and screw rings.

In a medium stainless-steel saucepan, combine the sugar, vinegar, mustard seed, celery seed, and turmeric. Add the vegetables and bring the mixture to a boil. Reduce heat to low, and simmer for 15 minutes.

Pack relish into the jars, leaving 1/2-inch head-space. Use a nonmetallic spatula to remove any trapped air bubbles, and wipe the rims clean with a damp cloth. Place on lids and screw bands, tightening only until fingertip-tight.

Process for 10 minutes in a boiling water bath.