Shochumimai/Zanshomimai (postcards)
In the same way as sending New Year cards (Nengajo), Japanese people send midsummer greeting cards called Shochumimai (暑中見舞 to friends and relatives. This derives from the custom of caring about each other’s health at the hottest time of year. The cards are normally sent during the period between the end of rainy season (late July) and August 7 (the day before Risshu, the first day of autumn). Cards can be sent after August 8, however the name changes to Zanshomimai. Just like the New Year cards printed by the post office, there are midsummer cards with lottery numbers on them for a draw in which recipients of the cards have a chance to win presents.
Postcards in summer (暑中見舞い and 残暑見舞い
ilovejapancul.blogspot.com/2008/08/postcards-in...
"Shochu" 暑中 means the hot period from the end of rainy season to Liqiu (August 7 in this year). and "Zansho" 残暑 means the heat after Liqiu. So we write "Shochu Mimai" before Liqiu and "Zansho Mimai" (We call each postcards as "Shochu Mimai " and "Zansho Mimai".)