It is said that unagi, or freshwater eel, has been consumed in Japan for thousands of years. Because it is rich in protein, vitamins A and E, and so on, some people believe that unagi gives them stamina. For this reason, Japanese people eat the eel most frequently during the hottest time of the year in Japan.
It is therefore a Japanese custom to eat unagi on Doyo-no-ushinohi (the Day of the Ox during the Doyo period) in summer sometime between mid-July and the early part of August.
Filleted and deboned, unagi is commonly glaze-grilled, a process that makes it a dish known as unagi-no-kabayaki. It is the skewered and grilled with sweet basting sauce and is typically served over steamed, white rice. Vacuum-sealed unagi-no-kabayaki is often available at Asian grocery stores.
Unagi-no-kabayaki is cooked differently in eastern and western Japan. In the eastern part of the country, it is generally steamed after being grilled, and then it's grilled again. In the western part of Japan, unagi is not normally steamed before grilling. One result is that unagi-no-kabayaki in eastern Japan is more tender than the same dish in western Japan.
Not to be confused its saltwater cousin, which is called anago, Unagi also can be used as an ingredient in other Japanese dishes like unagidon in which the eel is sliced and served on a bed of rice.
Sushi made with unigi is also pretty common fare. The sushi version is called unakyu.