Today is Eel Day in Japan. It's a summer holiday (sometimes two, depending on the calendar) where you eat eel to beat the heat.
It's rumored that a man owning an eel restaurant wanted to boost his sales so he consulted a friend. His friend recommended that he post an ad on a certain time during the summer. During that time it was believed to be good to eat food that began with the sound "u" (eel in Japanese is うなぎ, unagi) and his sales increased markedly.
There are other theories, but almost all talk about some sort of marketing ploy. Today people say that eel helps your "stamina" and keeps you safe from heat stroke. I disagree but it tastes good, so I had the bento above for lunch.