My husband thinks it is useless to water unless the sun is out because photosynthesis is not occurring. I say you can also water on overcast days, especially during our dry periods. He says there is no benefit on overcast days. (Rain is not forecast, it's just cloudy. We are in a drought.)
When to Water the Grass
Certified GKH Gardening Expert
You will be glad to learn that photosynthesis will occur at anytime there is any visible light present. This includes overcast weather. Also worth noting, is that watering and photosynthesis aren't necessarily connected as you would think. They can use water without photo synthesis, as they are still breathing all the time. This "breathing" is where most of the water is being spent, along with some being used for the photosynthesis cycle. It is best to water in the early morning, if possible.