It is the method to make the hardened futon soft. During the night in clean weather and in summer, air out a futon outdoors overnight to expose it to the nightly dew. And leave it outdoors during the day on the following day to expose it to the strong sunshine. By leaving it outdoors for a couple of days like this, the cotton batting of the futon becomes soft and fluffy. This yotsuyu-boshi method is hardly used today. However, it is said that yotsuyu-boshi was popularly practiced at some time from late July to early August each year in the past.