When is the best time to aerate a lawn?

FAQ
Question: 

When is the best time to aerate a lawn?

Answer: 

In Iowa, September and April are the best times to aerate Kentucky bluegrass and other cool-season lawns.  While the overall results are beneficial, core aeration causes some initial damage.  Aerating in September or April allows the grass to quickly recover during the favorable growing conditions in spring and early fall.