summer airing of clothes
A traditional practice of airing clothes, books, and other items during the hottest, driest period of summer (the 'doyo' period) to prevent mold and insect damage.
祖母は毎年、土用干しに着物を虫干しする。
Every year, my grandmother airs out her kimono during the summer airing season.
Compound of 土用 (doyo, the 18-day period before the start of each season, especially the summer one) and 干し (hoshi, drying/airing). The term refers to the custom of airing items during the summer doyo period.