Wash, peel and chop cucumbers. Measure 7 cups (1.75 L). Wash seed and chop green and red peppers. Measure 4 cups (1 L) each.
Combine cucumbers, peppers, celery, onions and pickling salt in a large glass, stainless steel or enamel bowl. Cover and let stand 4 hours.
Fill boiling water canner with water. Place 7 clean pint (500 mL) mason jars in canner over high heat.
Rinse vegetables well and press out excess moisture.
Combine sugar, vinegar, celery seed and mustard seed in a large stainless steel or enamel saucepan. Mix well; bring to a boil. Add vegetables; return to a boil and boil gently 10 minutes.
Place Snap Lids in boiling water; boil 5 minutes to soften sealing compound.
Ladle relish into hot jar to within ½ inch (1 cm) of top rim (head space). Remove air bubbles by sliding a rubber spatula between glass and food; readjust head space to ½ inch (1 cm). Wipe jar rim removing any stickiness. Center Snap Lid on jar; apply screw band just until fingertip tight. Place jar in canner. Repeat for remaining relish.
Cover canner; return water to a boil; process 10 minutes at altitudes up to 1,000 ft (305 m). Remove jars. Cool 24 hours. Check jar seals. Sealed lids curve downward. Remove screw bands; store separately. Wipe jars, label and store in a cool, dark place.