Teika is here referring to the nagoshi no harae (夏越の祓), a purification ritual carried out every year on the last day of the Sixth lunar month (towards the end of August) in order to avoid illness and ill fortune in the second half of the year. Despite what he says in the poem, it did not generally involve cleansing oneself with water: people would visit their local shrine and be purified by passing through a hoop of cogon grass.