This year the days to eat unagi (eel) are July 24th and August 5th. The days are called 土用の丑の日（Doyo no uchi no hi) - sometimes called Eel Day for lack of a better term in English. Actually, I didn't even know there could two days, but they are listed on the Japanese Wikipedia page. The story can be found lots of places on the internet, but essentially during the Edo Era unagi sellers came up with a story that if you eat unagi on the Ushi no Hi day of the Chinese calendar in during the summer it will help you beat the heat.
What I found interesting was that it is such a boost for unagi sales that this year Taiwan eel farmers are conducting a big push to get Japanese to eat Taiwanese eel, stressing the safety and traceability of Taiwanese cultivated eel.
Here is another interesting post about a new eel extract drink.