Getting a hint from the folklore that "one could overcome the summer heat by eating something that had the Japanese syllable of 'u' at the head of its name, such as an eel (in Japanese, 'unagi') on the Ox day," Gennai recommended that the owner attach a poster to the shop door stating, "Today is the Ox day."
源内は、「丑の日に『う』の字が附く物を食べると夏負けしない」という民間伝承からヒントを得て、「本日丑の日」と書いて店先に貼ることを勧めた。