Shochumimai (summer greeting card) (暑中見舞い)
A shochumimai is a letter sent to acquaintances to ask how they are doing in the mid-summer. It also means visiting the house of an acquaintance.
Summary
It means caring health of others in the hottest season of the year. The opposite of a kanchumimai (winter greeting card). It is sent from the end of the rainy season till risshu (the first day of autumn) and after that it changes to zanshomimai (late-summer greeting card).
Shochu (mid-summer) and zansho (lingering summer heat)
According to Nijushisekki (24 divisions of the old calendar), a greeting card written after the end of the rainy season in the destination area is called shochumimai. A greeting card written after August 8th is called zanshomimai.
Notes
Japan Post Holdings Co., Ltd sells Summer Greetings Postcards for shochumimai. There are plain post cards and those with printed illustrations. It has a lottery number to win a prize as New Year postcards.