There is a custom of eating eel on theis day. There is a custom of eating eel on this day.

In Japan, today is called "tThe mMidsummer dDay of the oOx." In Japan, today is called "The Midsummer Day of the Ox."

Typically, holidays are capitalized.

According to one theory, this custom first appeared in the Edo era. According to one theory, this custom first appeared in the Edo era.

Historically, Eel iwasn't a summer-seasonal food andbecause it was too fatty to eat in summer. Historically, Eel wasn't a summer-seasonal food because it was too fatty to eat in summer.

So someone started to advertise that eel was nutritious against summer heat. So someone started to advertise that eel was nutritious against summer heat.

Possibly, "eel had to nutrients to help against the summer heat" or "that eating eel would help combat the summer heat"

Nowadays, ithere is a problem that a lot of elels are overfished even though they are one of thean endangered species. Nowadays, it is a problem that a lot of eels are overfished even though they are an endangered species.

I'm not sure of the grammar, but this sounds more natural to me.

Nowadays, there is a problem that a lot of *eells* are overfished even though they are one of the endangered species. Nowadays, there is a problem that a lot of *eels* are overfished even though they are one of the endangered species.

Looks like you accidentally spelled it incorrectly. Everything else was perfect!