Put the cucumbers in a large container. Cover the cucumbers with salt water (about 1 tsp salt to 1 cup of cold water). Refrigerate for about 2 hours.
Drain soaked cucumbers, rinse thoroughly, and place into a large container with sliced onion. In a separate bowl, stir together Miracle Whip, sugar, and apple cider vinegar. Pour dressing over cucumbers and onion. Stir to combine.
Refrigerate salad for several hours to overnight. Flavors need time to mingle.