Combine zucchini, onion, peppers, and salt in a large bowl, stirring well. Chill overnight. Place in a colander to drain; rinse thoroughly and drain again. Transfer to a large stock pot and add remaining ingredients; bring just to a boil, then simmer for 10 minutes. At this point, the relish is done.
Add celery seed, nutmeg, turmeric, and black pepper. Stir to combine, then add drained zucchini. Bring to a boil over medium-high heat; reduce heat to medium-low and simmer 30 minutes. Meanwhile, sterilize seven 1-pint jars and lids. Pack relish into sterilized jars, making sure there are no spaces or air pockets. Fill jars all the way to the top.
Combine zucchini, onion and salt, let stand overnight, then drain and rinse in cold water. Add remaining ingredients, mix well and bring to a boil. Simmer 30 minutes, then ladle into hot pint jars and seal. Process in hot water bath for 15 minutes. Remove from canner and let cool on towel, check for seal after cool. Makes about 7 pints.
Set the jars in the canner, add water to cover, and simmer (180˚F) for 10 minutes. Keep hot until you are ready to fill them. Drain and rinse the zucchini mixture well with cold water to remove the extra salt. Add the sugar, celery seed, mustard seed, and vinegar to a large stockpot.