It continues hot days that exceed body temperature every day.
There was a custom of writing summer greeting cards to be concerned about your acquaintances' physical condition how they are doing well as they spend hot days.
Nowadays, we have useful tools such as cellphones, so we rarely send them to close friends, but they are still used for expression the appreciation to your superiors in a formal manner or occasionally used for advertising of shops.
In addition, there is a subtle deference of the Japanese words depending on the time when you send a card.
Even if the heat is like same, from the middle of August greeting sentence will be replaced the greetings of the worst part of the hot season with the greetings of the time that still remains terrible hot.
The summer greeting card we send till by about 10th of August is called "Shochu-mimai" in Japanese, it says" How are you? I'm anxious about you in these very hot days."
And one sent from mid-August is called "Zansho-mimai", it remarks "How are you? I'm worried about you though it still remains terrible hot days."
You don't send both, you adapt your manner of suit the words to the timing of sending cards.