Today, July 26, is doyo ushi no hi, which translates directly as "hot day of the ox" though the food that is traditionally eaten on this day is not beef but rather unagi (eel). So why isn't it called unagi no hi? That's a good question and one that isn't easy to answer, since the idea originated many centuries ago in China, where the cycle of days was based on 12 rather than 7, and to help people remember the cycle each day was assigned an animal, in the same way that the old calendar groups years into cycles of 12 animals. Somehow over time the day of the ox came to represent the high point of summer, meaning the hottest day of the year, which was pinpointed as being 18 days before the first day of autumn according to the old calendar.
This year that day falls on July 26, which won't make sense to anyone except people with an interest in arcane Asian astrology. It certainly doesn't explain why people eat unagi on this day. There are many theories, one of which is that both "ushi" and "unagi" begin with the "u" sound, though the hypothesis that seems to be the most accepted is that some time during the Edo Period (1603-1868) fishmongers got together to promote eel because it didn't sell as well as other fish and hit on the idea of saying that it boosted stamina during the dog days of summer. In other words, it was a marketing ploy, and just as American greeting-card companies effectively created Mother's Day, these fishmongers hit on the proverbial hottest day of the year — doyo no ushi no hi — as a good day to promote eel sales.
The Japanese populace bought it, and ever since then unagi sales have been brisk on this day, despite the relative high price compared to other fish. The high value attached to unagi is another mystery and, again, seems to be more a matter of marketing than anything else. In recent years, there have been many scandals involving unagi distributors who have purposely mislabeled unagi from Taiwan or China as having been grown in Japan. Domestic eel commands a higher price, even though in many cases there's absolutely no difference. The bulk of unagi sold in stores and to restaurants in Japan come from eel farms, and most of the farms, whether they are in Japan or China or Taiwan, get their fry from Japan.