Unagi (freshwater eel) has been consumed in Japan since the 17th century. Unagi is rich in protein, calcium, vitamin A and E, and it's said that unagi gives people stamina. For this reason, unagi is eaten the most during the hottest time of the year.
It's a Japanese custom to eat unagi on Doyo-no-ushinohi (the cow day during the summer Doyo period). The end of each season is named Doyo, according to a Chinese philosophy called gogyo, and there are four Doyo periods per year. It's defined by ecliptic longitudes of the Sun, so the dates slightly change every year. There can be one or two cow days during a Doyo period, and Doyo-no-ushinohi in summer falls between mid July and early August each year. It's known as eel eating day in Japan.
When unagi is grilled over charcoals, the fat from unagi drips and burns, causing smoke. The smoke adds a great flavor to unagi, and the great smell coming from unagi restaurants attract many customers into the restaurants every summer.
Unagi is commonly glaze-grilled, and it's called unagi-no-kabayaki. Boned and filleted unagi is skewered and grilled with sweet basting sauce (kabayaki sauce). It's similar to teriyaki. Vacuum-sealed unagi-no-kabayaki is often available at Asian stores. See Tips for Using Frozen Unagi
Unagi-no-kabayaki is cooked differently in the eastern Japan and the western Japan. In the eastern Japan, unagi is steamed after it is grilled, then it's grilled again. In the western part of Japan, unagi isn't steamed before grilling. So, unagi-no-kabayaki in the eastern Japan is tender than that of the western Japan. Unagi grilled without basting sauce is called shira-yaki. Shira indicates the color of white in Japanese.