If you buy a Japanese newspaper, then you will get stacks of advertising flyers inside the paper especially on Friday and Saturday. On Fridays and Saturdays the supermarkets complete especially hard for customers by offering discounts on "koukoku no shina", or "advertisement items", basically loss-leaders sold very cheaply to attract customers. Depending on what you want to buy, you can pick up stuff most cheaply at the weekend, especially if you can read the Japanese for koukoku no shina.