Lawns in the Northeastern United States benefit from one or two core aerations per year -- Spring & Fall -- with Fall being the preferred time of year if it is only being done once.
Having adequate soil moisture present while aerating will aid in the removal of longer and deeper soil cores, however, soil conditions should not be muddy.
Most equipment rental yards have core aeration machines and neighbors can go together on the rental cost to share the machine and save money.
Spreading grasses such as Kentucky Bluegrass have white roots known as rhizomes. This type of spreading grass tends to produce thatch faster than a "bunch-type" grass like Perennial Rye.
WHAT DO SOIL CORES LOOK LIKE?
Soil cores, pulled out by an aerator, provide a good cross-section of the lawn surface and soil profile.