土用の丑の日 is a traditional form used to describe a date, where 土用 means the period of 18-19 days before the start of a season (spring, summer, autumn, winter) to the start, and 丑の日 specifies the date (the second day, since 丑 is the second in the twelve earthly branches, i.e., 子, 丑, 寅, 卯, 辰, 巳, 午, 未, 申, 酉, 戌, and 亥). This method of date recording is based on seasons, which is quite tradtional and has little usage in nowadays Japan, one of them being these special dates related to holidays or other traditions like food.

So long story short, 土用の丑の日 is some day during midsummer, sometimes there are two (one in late July, one in early August), and sometimes only one in July. Even most native Japanese don’t understand how it’s determined, but they know it’s the day when all the eel rice (or in Japanese, unadon/鰻丼) restaurants remind them that it’s time to enjoy some eel.

This tradition of eating eel rice in the summer dates back to the 1700s. It was documented that, in order to lower the risk of health issues in the hottest time of the year, one needed to have nutritious food, and eel rice was believed to be of high nutrition. There are also a few other hypotheses regarding the origin of this tradition, but now it’s impossible to determine the genuine one. As of the most recognized saying of nutritious food, in modern times people in developed countries like Japan worry more of consuming too much nutrition instead. All in all, whatever reason it was that caused eating eel rice to become a tradition, it is unlikely that the reason prevails. You can read more about 土用の丑の日 on wikipedia in Japanese.

Still, tradition is tradition, and the consumption of eel rice does peak during this day. To have certain type of food on a certain day is often well established social norm in Japan, such as chocolate on Valentine’s Day, KFC on Christmas (it’s a long story, might talk about it some time, but still, whyyyyyy), and eel rice on the day of the ox in midsummer. Social norms are important in Japan, and business owners sometimes enthusiatically push consuming certain product on a certain date or during a certain period to become a tradition, cause it’s obviously good for business.

A bit more on eel. Eel rice is a delicious cuisine, but eels are becoming scarcer. Although it is possible to artificially raise an eel from baby eel, to let them reproduce under human-controlled environment is still mostly unattainable. Eels mature (sexually) under very delicate circumstances, and those raised in a pond would not have babies (well, in some labs researchers succeed, but the cost is ridiculously high for practical use). So currently the only source of eel that people eat is from catching eels in their infancy and feeding them in aquatic farms. An even more precious kind of eel is mature wild ones. Eel rice made from wild eel caught alive is the cherry of all kinds of eel rice.

To make things worse, the overfishing of eels has significantly reduced the amount of them. Now a great proportion of eel sold in Japan is actually imported from China (well the Chinese, too, feed from baby eels, but they catch different species from other areas). In the future there might be a day when we can no longer enjoy the original Japanese eel on this 土用の丑の日.

Oh, the reason 土用の丑の日 is trending is, as aforementioned, restaurants use this date to promote their eel rice, and consumers tweet about the food they just had.