Today,being the 21st of July,is the most important unagi (eel) day in Japan.
In ancient times people in Japan used the koyomi (almanac) consisting of 12 animal characters (the Chinese lunar calendar) in those times,they used it for time as well as months and days. (ie) (the hour of tiger,ox,rabbit etc) We call those characters "eto",nowadays there are only a few vestiges of this ancient almanac in modern Japanese culture.
People still remaining their own Chinese zodiac character, we learn them at school the age of 10.
Anyway according to the koyomi , today is a "doyo-no-ushi" there are 5 or 6 such days a year and when people hear the word,almost everyone in Japan thinks of eating unagi (eel). It's part of food culture to eat eel on "doyo-no-ushi".
There is an old Japanese book written in 1822,which tells the story of dodgy eel merchants coming up with a scheme to sell a lot of eel on these days,especially in summer,when people tend to lose their appetite.According to the book,eel merchants consulted a scholar about their low sales in summer and possible means to combat this problem in their business.
Unagi is vitamin rich,it's effective for combatting "natsu-bate"(summer fatigue) and beginning with "U" it was suggested to promote it's benefit its on "doyo-no-ushi".
This morning I received a lot of fliers,in my daily newspapers.Every supermarket,convenience store,or restaurant tries to take advantage of this custom to promote sales of unagi on this day.Of course I ate unagi as my dinner.