Shochū mimai, the midsummer greeting card, is sent from the end of the rainy season until the day before risshū, the start of autumn in the traditional calendar. In 2024, that day was August 6. After it, the card becomes zansho mimai, a late-summer greeting, and should arrive by the end of August at the latest.
An etiquette guide gives a business version. It opens by wishing the recipient’s company ever greater prosperity.