Wu Xing is a theory that all things in this world consist of five elements, Wood[木], Fire[火], Soil[土], Metal[金], Water[水]. As for seasons, five elements were applied each to spring(Wood), summer(Fire), autumn(Metal), winter(Water) and Doyō[土用](Soil). Doyō are periods during about 18 days before a day of starting each season. First day of season is each called first day in spring: Risshun[立春(りっしゅん)](about Feburuary 4th), summer: Rikka[立夏（りっか）](about May 5th), autumn: Risshū[立秋（りっしゅう）](about August 7th), winter: Rittō[立冬（りっとう）](about November 7th). Doyō is a preparation period of each season.
Ushi-no-hi[丑の日（うしのひ）](ox day) is a day of Jyūni-shi[十二支（じゅうにし）](the Chinese Zodiac) that was a thought born in ancient China. Jyūni-shi expresses times and directions in 12 animas. For example, as for days, Jyūni-shi goes round in 12 days and the next day's animal is the same one in the first day. Ushi no hi is second day in Jyūni-shi. Naturally, an animal of a day is not same each year.
Sometimes, Doyō has two Ushi-no-hi in each period because Doyō is 18 days and Jyūni-shi is 12. In this case, the second Ushi-no-hi is called "Ni-no-ushi[二の丑（にのうし）]".
In modern Japan, Doyō-no-ushi-no-hi often indicates a day during summer period, in other words, Ushi-no-hi before the day of Risshū. There is a custom that Japanese eats eel in this Ushi-no-hi.
According to one theory, in the Edo Period(1603-1868), an owner of eel restaurant was worried because there are few customers in summer.(The season of eel is winter and it tastes worse in summer.) The owner consulted with an inventor, also scholar, writer and painter, Gennai Hiraga[平賀源内（ひらがげんない）]. Gennai advised him to post a sign "Today is Doyō-no-ushi-no-hi " and the owner followed his advice. Thanks to that, the restaurant prospered. Behind this, it was thought that a food that the word starts with "う(U)" in Ushi-no-hi was effective for summer heat fatigue.
Summer eel is not the best in summer, but an eel is rich of vitamin A & B that are effective for summer heat fatigue.