Why Do Japanese People Eat Eel in the Middle of Summer?

The short answer

In Japan it is a custom to eat kabayaki, eel grilled with a sweet and savoury sauce, on the summer "Doyo no Ushi no Hi" (the Day of the Ox in the doyo period, explained below). The idea at its root is to eat something nourishing to fend off natsubate, the exhaustion that comes with the summer heat.

The notion that eel helps when the summer leaves you thin already appears in the Man'yoshu, a poetry anthology from the Nara period. On the other hand, the famous story that the Edo-period inventor Hiraga Gennai popularised "eel on the Day of the Ox" has, in fact, no historical records to back it up.

What is the Day of the Ox in doyo?

According to Japan's Ministry of Agriculture, Forestry and Fisheries, doyo is the period of about 18 days just before risshun, rikka, risshu and ritto, the days traditionally counted as the start of spring, summer, autumn and winter. In the old calendar, days were also assigned the twelve zodiac animals (rat, ox, tiger and so on) in a 12-day cycle, and a day of the Ox that falls within a doyo period is a Doyo no Ushi no Hi.

That means there are actually several of these days every year. The eel-eating one, though, is in the summer doyo before risshu (the start of autumn), which usually lands around late July. According to the calendar of the National Astronomical Observatory of Japan, the summer doyo in 2026 began on 20 July. Because an Ox day comes every 12 days and doyo lasts about 18, some years have two summer Ox days, and the second is called "ni no ushi" (the second Ox).

"Eel for summer thinness", 1,300 years ago

The Japanese have been eating eel for a very long time: eel bones have been dug up from shell mounds across the country. The oldest text said to describe eel as good for you is the Man'yoshu. Otomo no Yakamochi, the central figure in compiling it, wrote a poem teasing a thin acquaintance, roughly saying "I hear eel (then called munagi) is good for summer thinness, so go and catch some and eat it."

So why the Day of the Ox? There are several theories. One is an old saying that it is good to eat foods starting with "u" (as in unagi, eel) on an Ox (ushi) day. Another comes from the archaeologist Higuchi Kiyoyuki, who said that eating black foods on Ox days was considered good, and eel was one of them. Either way, the custom of eating something energising in the summer doyo seems to go back well before the Edo period.

Was it really Hiraga Gennai's advertising slogan?

The story usually told goes like this: an eel restaurant whose customers dropped off in summer asked Hiraga Gennai for advice, put up a sign outside saying "Today is Doyo no Ushi no Hi", and business boomed. Gennai also worked on adverts for tooth powder and is sometimes called Japan's first copywriter.

However, according to a column in the Yomiuri Shimbun newspaper, there are no historical records confirming that Gennai wrote this slogan. Similar stories are also handed down: that the comic poet Ota Nanpo wrote a promotional verse at an eel shop's request, or that it all began when an eel shop in Kanda, Edo, received a huge order from a feudal lord. Nobody knows which, if any, is true. Enjoy it as a good story, but it is worth knowing that it is not the accepted account.

Incidentally, wild eel is said to be truly in season from autumn to early winter, when it builds up reserves for the cold. Eating it in high summer is less about peak flavour and more about taking part in a seasonal tradition.

Nutrition, and a few kabayaki facts

According to the Ministry of Agriculture, Forestry and Fisheries, eel is rich in vitamins A, B1, B2, E and D, as well as calcium, iron, zinc and fats such as DHA and EPA. Vitamin A in particular: 100 grams gives an adult their full daily requirement.

The name kabayaki has a story too. A well-known theory is that eel used to be skewered and grilled whole without being opened out, and its shape looked like the flower spike of the gama (cattail, or bulrush), a waterside plant, so it was called "gamayaki". Records from the Muromachi period describe eel cut into chunks, skewered and grilled, and Heian-period aristocrats reportedly ate it steamed. In the Kansai region eel is sometimes called "mamushi", said to come from "mamushi" written as "steamed in between", because the eel is tucked between layers of rice and steamed.

Kabayaki is classically sprinkled with sansho, a spice with a tingling, numbing heat. It is said this began as a way to mask the strong smell of farmed eel in the old days. The way eel is opened also differs by region: in the Kanto region it is usually opened from the back, in Kansai from the belly. A well-known explanation is that in Edo, a city full of samurai, cutting the belly was disliked because it recalled seppuku (ritual suicide by cutting open the belly), while in the merchant city of Osaka eel was opened from the belly so that people could "open their bellies", a Japanese expression for talking frankly.

There is also an old saying that eel and umeboshi (pickled plum) are a bad combination, but it has no medical basis. One theory is that it was a warning: umeboshi boosts your appetite, so it stopped people eating too much expensive eel. Eel blood does contain a toxin, but it is a protein that breaks down with heat, so well-cooked kabayaki is nothing to worry about.

Japan is not the only place that eats eel. According to the Ministry of Agriculture, Forestry and Fisheries, in China and Taiwan it is steamed or stewed with aromatics, in Spain baby eels are sautéed with garlic and olive oil, and in Denmark and Germany eel is smoked, so Europe has its own eel-cooking culture too. Still, a whole country eating eel on one particular day does seem to be uniquely Japanese.

The eel shortage you should know about

The eel on Japanese tables actually faces a serious problem. In 2013 Japan's Ministry of the Environment listed the Japanese eel as Endangered IB (a species at high risk of extinction in the wild in the near future). Based on catch data for wild eel, it calculates a decline of 72 to 92% over three generations. It is also listed as endangered on the Red List of the International Union for Conservation of Nature (IUCN).

Most eel sold in Japan is farmed, but the technology to raise eels from eggs has not yet been commercialised, so farms catch wild juvenile eels (glass eels, called shirasu unagi) and grow them. The Fisheries Agency is developing technology to mass-produce artificially bred juveniles. In December 2025, at a conference of the parties to CITES (the Washington Convention, which regulates international trade in wild animals and plants), a proposal by the EU to tighten trade controls on all eel species was rejected, but how to protect eel stocks remains a big challenge.

A wild eel takes more than ten years to reach adulthood, while farms grow them to market size in about six months. To be ready for the Day of the Ox, glass eels are caught intensively around six months beforehand. Some years ago, when the Ministry of the Environment posted on social media ahead of the Day of the Ox urging people to "book ahead and eat it with care", it was flooded with criticism along the lines of "Are you recommending we eat an endangered species?", and the post was deleted.

Some experts point out that concentrating demand on the Day of the Ox drives the overfishing of glass eels. If you do eat eel, booking ahead so that nothing is wasted matters too.

Tips

Eel restaurants are packed on the summer Day of the Ox. If you want to eat at a popular place, book ahead or go on a different day.
Give your kabayaki a shake of sansho. Its aroma and tingling heat cut through the richness.
Eel for kabayaki is prepared differently in Kanto and Kansai (opened from the back in Kanto, from the belly in Kansai). Comparing the two on your travels is part of the fun.
Eel is a fish whose extinction is a real concern. When you eat it, order only what you need so none goes to waste.