The 300-Year-Old Marketing Campaign That Became a National Ritual

Every July, Japan sells millions of portions of grilled eel on a single day. Lines stretch out of specialty restaurants, supermarket shelves empty by noon, and families gather around steaming bowls of rice topped with glossy, caramelized fillets. The remarkable part? This national obsession started as an 18th-century advertising campaign.

Hiraga Gennai (1728–1779), an Edo-period inventor, author, and polymath, is widely credited with the idea. When a struggling eel restaurant owner asked for advice, Gennai suggested posting a sign promoting eel as a stamina-boosting food on the Day of the Ox. The pitch worked spectacularly. The catch? Eel isn't actually in peak season during summer. The tradition was born from clever marketing, not natural seasonality.

That campaign became Doyo no Ushi no Hi, a cultural ritual that has endured for over 300 years. Read on for the full history, the food itself, and how to make it at home.

What Is Doyo no Ushi no Hi? Understanding Japan's Midsummer Day of the Ox

To understand this tradition, you need to know two Japanese terms. Doyo refers to the roughly 18-day transitional period before each season change in the Japanese lunisolar calendar. There are four Doyo periods throughout the year, but the summer Doyo, falling between July and August, is considered the hottest and most physically demanding stretch of the Japanese calendar.

This is when natsubate, or summer fatigue, sets in. Think of that bone-deep exhaustion that builds after weeks of relentless heat. It's a universal experience, and the Japanese gave it a name centuries ago.

Ushi no Hi means the Day of the Ox, a date calculated using ancient Chinese Daoist philosophy rooted in Yin-Yang theory and the Five Elements. In some years, two Days of the Ox fall within the summer Doyo period, meaning Japanese families may eat unagi twice. In 2026, Doyo no Ushi no Hi fell on Sunday, July 26.

Eel isn't the only traditional food for this day. There's a broader custom of eating "u foods" to ward off summer heat: udon, umeboshi (pickled plum), and uri (a category that includes gourd, cucumber, and watermelon). The folk belief held that foods beginning with the syllable "u" offered protection against the punishing summer climate. If you've ever craved cold noodles or watermelon on a sweltering afternoon, you already understand the instinct behind this tradition.

Why Eel? The Nutritional Science Behind the Tradition

Unagi has been consumed in Japan for over 1,200 years, making it one of the country's oldest culinary traditions. Long before anyone could measure vitamins, Japanese people observed that eating eel made them feel stronger during the summer months. Modern nutrition science suggests they were onto something real.

A 100-gram serving of grilled unagi provides approximately 184 calories, 18.4 grams of protein, and 11.5 grams of fat. It's also rich in vitamins A, B12, D, and E, along with omega-3 fatty acids. These nutrients support energy production and immune function, exactly what your body needs when heat stress is wearing you down.

If you've seen eel at a sushi restaurant, you may have noticed two types on the menu. Unagi is freshwater eel, with a high fat content and a rich, buttery flavor. Anago is saltwater (conger) eel, leaner with a milder, fluffier texture. The two are often confused by American diners, but they're quite different in taste and preparation. When Japanese people talk about eating eel on Doyo no Ushi no Hi, they mean unagi.

A standard Unadon serving, with about 150 grams of eel over a cup of rice, comes in at roughly 500 to 600 calories. It's a complete, satisfying meal in a single bowl.

Kanto vs. Kansai: Two Ways to Grill an Eel

Japan has two major regional styles of preparing unagi kabayaki, and the dividing line runs near Hamamatsu city in Shizuoka Prefecture. This is one of those friendly culinary rivalries that gets people genuinely passionate, much like the regional barbecue debates you'll find across the American South.

Kanto style (the Tokyo region) calls for splitting the eel down the back, then steaming it before grilling. The result is a soft, fluffy fillet that practically melts on your tongue. The steaming step renders out some of the fat and creates an exceptionally tender texture.

Kansai style (the Osaka and Kyoto region) takes a different approach. The eel is split down the belly and grilled straight through without any steaming. This produces crispier skin, a firmer bite, and deeper caramelization from the direct heat.

Both styles use the same sweet-savory tare sauce, a glaze made from soy sauce, mirin, sake, and sugar. The sauce is applied in repeated layers during grilling, building up a lacquered, aromatic coating. Pre-grilled frozen unagi sold in the US is almost always prepared in the Kanto style, so you're getting that classic soft, steamed-then-grilled texture.

How to Make Unadon at Home: A Simple Guide for American Cooks

Here's something that surprises many people: Japanese home cooks don't prepare live eel. They buy pre-grilled, vacuum-sealed unagi fillets and reheat them at home. This is the authentic home-cook approach, not a shortcut. Starting with a frozen fillet means you're doing exactly what families across Japan do every summer.

When shopping for unagi in the US, quality varies significantly. Japanese-imported fillets, often sourced from Kagoshima Prefecture, typically cost $30 to $40 per fillet. Frozen eel from other countries runs closer to $10. The price difference reflects real differences in flavor, texture, and sourcing standards. With over 20 years of experience sourcing authentic Japanese specialty products, we at Tomato Japanese Grocery can tell you the quality gap is noticeable.

Reheating is straightforward:

1. Remove the fillet from its packaging.
2. Optionally brush it with bottled tare sauce for extra glaze.
3. Warm it in a pan over medium heat, in a 350°F oven for 8 to 10 minutes, or briefly under a broiler until the surface is glossy and heated through.

Served over a bowl of steaming Japanese short-grain rice, this is Unadon. Plated in a lacquered box instead, it becomes Unaju. Both versions are equally simple to prepare at home.

To complete the meal authentically, serve it alongside a clear soup (suimono), Japanese pickles, and a sprinkle of sansho pepper over the eel. Sansho adds a distinctive citrusy, slightly numbing finish that cuts through the richness of the tare. Bottled tare sauce and sansho pepper are available at Japanese grocery stores across the US, including here at Tomato Japanese Grocery, both in-store in Marietta, GA and via nationwide UPS shipping.

Sustainability and the Future of Unagi

Any honest conversation about unagi needs to address sustainability. The Japanese eel (Anguilla japonica) was listed as Endangered by Japan's Ministry of the Environment in 2013 and added to the IUCN Red List of Threatened Species in 2014. This is a species under real pressure.

The numbers tell a clear story. Japan's unagi imports peaked at 133,000 tons in 2000. By 2021, consumption had fallen roughly 63%. In 2024, Japan consumed approximately 61,000 tons of eel, with nearly three-quarters of that imported from China and Taiwan.

The debate continues at the international level. In 2025, the EU, Panama, and Honduras proposed listing all 17 eel species under CITES to restrict international trade. Japan opposed the proposal, citing its own conservation measures and a Tokyo University of Marine Science and Technology study claiming that the Japanese eel population has tripled across East Asia since 1990. The proposal did not pass.

There are also concerns about what's actually in the package. A 2025 peer-reviewed study published in Fisheries Science found that 5.2% of products labeled as unagi in Japanese retailers were actually European eel, a critically endangered species. Read labels carefully and buy from trusted sources.

For those looking to participate in the tradition more sustainably, plant-based options are emerging. Nissin Foods re-released its "Nazo Unagi" in July 2024, a three-layer soy-protein product developed with chef Jun Kurogi that replicates the texture and flavor of kabayaki without any animal ingredients.

As a consumer, you can make informed choices: look for country-of-origin labeling, ask your Japanese grocery store about sourcing, and consider plant-based alternatives as a way to honor the tradition while supporting conservation efforts.

Bring the Tradition to Your Table This Summer

A 1,200-year-old food. A 300-year-old marketing campaign. A tradition that millions of Japanese families still observe every July. And all of it is more accessible to American home cooks than you might think.

Pre-grilled unagi, bottled tare sauce, sansho pepper, and Japanese short-grain rice are all you need for an authentic Unadon at home. To explore the broader Doyo no Ushi no Hi tradition further, udon and umeboshi are wonderful, budget-friendly ways to connect with Japanese food culture during the hottest weeks of summer.

At Tomato Japanese Grocery, we carry the authentic ingredients to make this tradition your own. Stop by our store in Marietta, GA, choose local pickup, or order online for nationwide UPS shipping. Browse our selection of unagi, tare sauce, sansho pepper, and Japanese pantry staples to get started.