Why do we hear crickets sounds in time before middle day in the end of summer in middle northern hemisphere?

Insects are not able to generate their own body heat and as such rely upon other heat sources for their continued survival. When they are cold they go into a form of sleep or hibernation. The main natural source of heat on the earth is the sun so insects will be awake and active when the weather is warm. In spring and it is fairly warm from midday and into the evening, in summer it is warm apart from late night and early morning and in autumn (or fall) it is warm only in the morning and early to mid-afternoon. That would be why you can hear the grhoppers at those times..