Why Does All of Japan Eat Eel in the Summer? (The Edo Period Marketing Genius)

Last updated: March 20, 2026

The Summer Fatigue

Summer in Japan is brutally hot and humid. It is so exhausting that there is a specific word for it: “Natsubate” (summer fatigue). To survive this draining heat, Japanese people turn to a very specific, luxurious comfort food: grilled freshwater eel, or “Unagi.”

The Day of the Ox

Every year in late July, Japan celebrates “Doyo no Ushi no Hi” (The Day of the Ox). On this specific day, almost every supermarket and restaurant in the country pushes massive displays of Unagi covered in sweet, sticky soy glaze. But why eel, and why in the middle of summer?

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The Ultimate Marketing Strategy

It is actually the result of a brilliant business strategy from the Edo Period (1603-1867)! An eel restaurant owner was struggling to sell his fatty fish during the hot summer months. He asked a famous inventor and writer named Hiraga Gennai for advice.

The Sign That Changed History

Gennai simply told him to put up a sign outside his shop saying, “Today is the Day of the Ox: Eat Eel!” Because of an old superstition that eating foods starting with the letter “U” on the Day of the Ox (“Ushi”) brings good health, the restaurant sold out instantly. The trend spread nationwide and never stopped.

A Delicious Business Hack

The marketing trick worked perfectly, and luckily, it turns out eel is naturally packed with vitamins and protein, actually helping to cure summer fatigue! It is a brilliant, centuries-old business hack that became one of Japan’s most delicious summer traditions.