Did you eat eel on the 22nd? Yes — it was Doyo no Ushi no Hi, the Day of the Ox in midsummer. Palaceside Building used to have an eel restaurant, but times change; it closed in spring, and it is a shame we cannot eat there anytime. Good places near Kudanshita exist, but I hesitated in the heat and missed eel on the 22nd. Others may be in the same boat. This year is fine, though — August 3 is also Doyo no Ushi no Hi.
Preaching to the choir, but Doyo comes from China's "Five Elements" (wood, fire, earth, metal, water, to which all things correspond) — one of the calendar's minor seasonal markers (days marking seasonal change aside from the five seasonal festivals and twenty-four solar terms). In the Five Elements, spring is wood, summer fire, autumn metal, winter water; the remaining earth element is assigned to seasonal transitions and called Doyo — about eighteen days just before Rikka, Risshu, Ritto, and Rissun.
"Ushi no hi" is a day when the zodiac sign is Ox (Ushi); the cycle turns every twelve days, so during Doyo (about eighteen days) it comes once or twice. Doyo occurs four times a year, each with an Ox day, but usually "Doyo no Ushi no Hi" means the summer one.
This year has two Doyo no Ushi no Hi: July 22 is "I no Ushi (no hi)," August 3 is "Ni no Ushi." I have personally resolved to eat eel on August 3 no matter what.
Why eel on Ox day? Origins are said to be Edo-period; the leading theory is that Hiraga Gennai (1728–1780), Edo-period rangaku scholar and inventor sometimes called Japan's Leonardo da Vinci, wanted to help an acquaintance's eel shop and posted "Today, Doyo no Ushi no Hi" — business boomed and other shops copied. Another theory says Shakusonjin (Ota Nanpo, 1749–1823), a bunjin and kyoka master of the Tenmei era, coined a catchphrase kyoka: "Eat eel on Ox day and it becomes medicine."
Truth aside, "ushi no hi" and "unagi" sound alike, and there is a tradition that eating things starting with "u" on Ox day helps you beat the summer — not only eel but pickled plum, melon, udon, horse meat (umaniku) in various regions (or used to be).
Either way, eating eel rich in vitamins A and B1 to get through summer makes nutritional sense.
Still, many ask whether eel tastes good in this season. Fish is generally best in autumn and winter when it is cold, fatty, and firm — hard to argue. In Okaya, Nagano Prefecture — an eel town on Lake Suwa — the chamber of commerce and others organized "Okaya Eel Town Association," and eel restaurants call out: "Eel's season is winter. On this 'Cold Doyo Ox Day,' eat 'Okaya cold eel'" (website "http://www.okayacci.or.jp/unagi/doyo/").
Well, freezing technology has improved, so it is delicious anytime, I suppose.