How often should I aerate a lawn?

FAQ
Question: 

How often should I aerate a lawn?

Answer: 

The frequency of aeration is largely determined by the soil type and the amount of use.  Lawns growing in heavy, clay soils and those subject to heavy foot or pet traffic should be aerated twice a year.  Once a year should be sufficient for lawns that are established on well-drained soils and experience little traffic.  September and April are the best times to aerate Kentucky bluegrass and other cool-season lawns in Iowa.