A while ago, I wrote in an article that traditionally Japan has many days on which we are set to eat certain food items. 土用（どよう）の丑（うし）の日（ひ）is another one of those and on that day people are to eat grilled eels.
土用（どよう）の丑（うし）の日（ひ）is not a set date every year. It is the Ox day during the transition period between spring and summer and this year it is 20 July, the day after tomorrow.
Japan is known for its hot, humid summer and lots of people lose appetite during that exhausting season. In order to stay healthy during that period, they developed a custom to eat nutritious eels.
If you are in Japan during this season, try it!