Summers in Japan are nothing to sneeze at. The sun is hot, but the humidity is stifling. Since the Edo Period (1600-1868), there is a custom in Japanese culture that during the hot summer season, one can get relief from the summer heat and increase stamina by eating a kind of freshwater eel called unagi (うなぎ) on a certain day called doyō no ushi no hi (土用の丑の日). This roughly translates to “Day of the Earth Ox”.
The Day of the Ox was based on the old Chinese-style calendar, and its yearly relation to the sun’s position, so the day changes from year to year. Further, 57% of the years have two such days called ichi no ushi (一の丑) and ni no ushi (二の丑). For years with only one day, it’s just called ichi no ushi (一の丑). For example, the year 2020 has two days: July 21st and August 2nd, while for 2021, it only falls on July 28th, and 2022 again will have two days: July 23rd and August 4th.
But why do people eat unagi (glass eels) on the Day of the Ox?
According to my old cultural guidebook, the specific tradition of eating unagi on doyō no ushi no hi began with a fellow by the name of Hiraga Gennai who was a kind of renaissance-man in Edo-Period Japan. The story goes that a local eel-seller in Edo (modern-day Tokyo) came to Hiraga because he wanted to increase slumping sales during the hot summer period. Hiraga made him a sign to present at his shop advertising that “Today is Day of U“, a Japanese pun.
The book states that in ancient Chinese thought, each season of the year would be subdivided into the five elements: earth, fire, water, wood and metal, and of these earth was assigned to a special 18-day period at the end of each season called doyō (土曜, day of earth). With four seasons, there were four doyō periods, and of these, the most famous of these doyō days began around July 20th during the hottest days of the year, which also happened to be traditionally assigned to the zodiac sign of the Ox.
So, the point of all this is that in the olden days, the Ox, or ushi (丑) was synonymous with the summer heat, and especially around the doyō period around July 20th.
Since unagi also starts with the Japanese letter “u” (う), Hiraga tried to draw a connection between the two. Other foods eaten around this time traditionally to improve stamina in the heat are freshwater clams, mochi rice cakes and eggs, however unagi is by far the most popular. The book also implies that the vitamin B content in unagi may actually help with fatigue and lack of appetite, so there may be some truth to it, but the true origins are a clever word-play by Mr. Hiraga to help a friend generate some business. 😉
P.S. I’ve tried unagi before, and it’s OK. It’s just not for me, but my wife loves it, and insists it is better in Japan due to freshness and quality. Between the taste and the increasing scarcity of unagi though, I’m not interested.