Combine cucumbers and salt in a large, shallow bowl; cover and chill 1½ hours. Move cucumbers into a colander and rinse thoroughly under cold water. Drain well, and return cucumbers to bowl. Add onion to the bowl and toss with the cucumbers.

Combine the granulated sugar, white vinegar, apple cider vinegar, brown sugar, mustard seeds, celery seeds and ground turmeric in a medium saucepan; bring to a simmer over medium heat, stirring until the sugar dissolves. Pour the hot vinegar mixture over cucumber mixture; let stand at room temperature 1 hour. Cover and refrigerate 24 hours. Store in an airtight container in refrigerator up to 2 weeks.