Toss zucchini, onions, and celery well with the canning salt in a large bowl. Let stand overnight or 8 hours.
Drain into a colander and rinse 3 or 4 times to make sure all of the salt is off of the zucchini mixture. Press by hand to get rid of any excess moisture.
In large pot, mix dry ingredients well then mix with vinegar and add zucchini mixture and the green and red peppers. Bring to a boil and simmer for 1/2 hour on med low heat, stirring occasionally. I use both white and apple cider vinegar.
Ladle into sterile jars. Run a thin utensil through relish to get rid of any bubbles. Wipe rim clean with a wet paper towel. Place a sterile lid and then screw on ring. Repeat until you have filled 7 pints. Process in hot water bath for 30 minutes. Enjoy!