In the old calendar, there was one "earth use" for each of the four seasons, so there should be four "earth uses" in a year. However, nowadays, the term "To-usagi" generally refers to the "summer season". To put it more carefully, the period of 18 days from the beginning of autumn (around August 8 on the Gregorian calendar) is considered to be a period of "earth use". The "Uchi no Iri" means the first day of "Uchi", which is around July 21 every year

Also, it is said to be the reason for the "USHI no HI" (the day of the ugly), which means that in order to avoid being defeated by the scorching heat of the summer, it is customary to eat things beginning with the "U" sound, such as "Uri" (melons), "Ushi" (beef), "Unagi" (eel), and so on. It was customary to eat things that begin with the "U" sound, such as "URI" (melon), "USHI" (beef), and "UNAGI" (eel). Later, in the Edo period, a scholar named Hiraga Kennai wrote a sign saying "Today is the day of the ugly" and hung it in front of the owner's store in order to help an eel restaurant that was not doing much business. The sign, which had no special significance, became the best advertisement to attract a large number of customers. ...... So the custom of eating eel rice on the "day of the earth's use of uglies" has survived to this day

Do you get it? It doesn't matter if you don't get it. For me, the "day of the earth's use of uglies" = "the day when you eat good eel rice"