Combine the zucchini, peppers, onions and celery in a non-aluminum bowl. Sprinkle with the salt and mix well.
Cover the vegetables with cold water and allow to stand for an hour or so.
After an hour, drain the vegetables well, pressing down with a spoon to squeeze out the remaining water. Set aside while you prepare the brine.
Combine the sugar, vinegar, celery seed, mustard seed and dry mustard in a large, non-aluminum sauce pan. I use a four quart enameled sauce pan.
Bring all that to a boil, then add the vegetables and return to a boil.
Reduce the heat and allow to simmer for a while until the relish becomes quite thick.
Ladle into a clean pint sized jar (about two cups) and store in the fridge.