Question
midwest lawn in missouri.  when is the best time to aerate?  

Answer
Best time is early fall (September) and next best time is spring (May).

Aerate when the soil has dried a bit as too wet soil will clogg the thines. Do not wait until the soil is too dry, however, as the stress on the lawn is worse then, and the thines may not penetrate deep enough into the soil. The best time is a lawn which has stayed pretty dry, but been watered thoroughly and deeply a few days to a week before.

Early fall is usually the ideal time as air temperatures will progressively get lower and thus reduce evaporation from the soil. Avoid core aerating during hot summer months.

Core aerating is extremely beneficial to the lawn. It eliviates compaction which permit grass roots to grow deeper and expand. it permits oxygen, water and nutrients (fertilizer) to reach the root zone and allow carbondioxide to escape (gas exchange).