How is the behavior of the characters linked to the hottest day of the summer ?

They are reacting more and more passionately; heat can often be a sign of passion. They are also looking for ways to entertain themselves when, in fact, it is very difficult to find entertainment when it is so hot and there seems to be no relief in sight. Such desire to find "something to do" makes them act in somewhat irrational ways, sort of in the "heat of the moment."