"Doyo ushi no hi" is a day in late July dedicated to eating eel. The day falls on July 30th this year. The custom began in the 18th century. It has been said since Japanese summer is hot and humid, many people suffer from natsubate (summer fatigue). It is believed that eating nutritious eel helps to increase stamina and beat the summer heat.
On Doyo ushi no hi, you will find eels sold in every supermarket. I'm sure, also Japanese restaurants in Düsseldorf serve grilled eel dishes on the day. Did you find "Long advertisement flags (nobori)" around the restaurant to promote sales??
I love eel. Una-jyu*, it's the one of dishes that I always eat when I go back to Japan. It is particularly good with sansho Japanese pepper. I feel hungry!
*Una-jyu is a Japanese cuisine consisting of unagi-kabayaki (eels broiled with a thick salty & sweet sauce) served over the cooked rice in a lacquered ware box.