Answer: Realistically speaking you can't aerate when the ground is frozen, excessively wet, or bone dry and hard. Midsummer is not a good time because the lawn is already stressed by heat and drought and will not recover well. The best time to do a core aeration is in late summer to early fall just before the lawn undergoes a natural growth period. Unless you have deep thatch or the soil is excessively compacted, you may not need to do this. Here is more information on aerating the lawn you may find helpful.
I hope this answers your question.
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