An "overcast day" refers to a day when the sky is covered in clouds, and the sunlight is partially or completely blocked. It is a gray and gloomy day. Full definition
They would generally photograph on overcast days in the spring and fall, which provided them an even light with a vast tonal range. (fraenkelgallery.com)
In London, a solar panel produces 65 times less energy on a heavy overcast day in December at 10 am than on a sunny day in June at noon. (lowtechmagazine.com)
«Hard Sun» has many of the things you expect from a British crime drama: Competent visuals, a solid cast, a serialized tale of criminality, and a nearly unending string of overcast days. (variety.com)