A sad day refers to a day where one may feel unhappy, down or depressed. It could be due to personal reasons such as loss, disappointment or stress; or it could also be because of external factors like bad weather, financial troubles or world events that affect people's emotions and outlook on life. In any case, a sad day is characterized by feelings of melancholy, despair or hopelessness, which can make the day seem long and difficult to get through.