残暑

Noun

 * 1) late summer heat, lingering heat (after, approximately August 8th)

Usage notes
Particularly used in letters and on greeting cards, especially in August, and into September.

According to the traditional Japanese lunisolar calendar, autumn begins on, which is approximately August 8th, and thus traditionally this term refers to “summer heat lingering into autumn”. However, since August in Japan is hot, indeed often the hottest month of the year – see and climate of Kyoto – August is today often referred to as “late summer”, while September is “early autumn” (traditionally it is mid-autumn).