Squash plants are pretty hardy plants, and the hard part is figuring out exactly what is wrong. If the plant is stressed, then the leaves start yellowing any time. Reason could be lack of water, Iron Deficiency or Bacterial Wilt. Squash need about two inches of water a week sometimes they need more due to high temperatures. Make sure that you don't overwater your plants. Add some Iron Chelates to the soil it's a kind of fertilizer. Usually, Iron deficiency is a result of the nutrients being leeched out of the soil due to over watering