midsummer day of the ox
Refers to the day of the ox during the midsummer doyō period, traditionally associated with eating eel to beat the summer heat.
This year's midsummer day of the ox is July 20th.
There is a custom of eating eel on the midsummer day of the ox.
土用 refers to the 18-day seasonal transition period, while 土用の丑の日 specifically denotes the day of the ox within that period.
丑の日 is a general term for any day of the ox in the traditional calendar, whereas 土用の丑の日 is the specific midsummer day associated with eating eel.
Compound of 土用 (doyō, 'seasonal transition period') + の (no, possessive particle) + 丑の日 (ushi no hi, 'day of the ox'). The association with eating eel is said to have been popularized in the Edo period.