Some will say aerating should be done in the Spring, some in the Fall. Some will even do both!
Personally, I prefer the Spring for aerating as it ensures soft, receptive soil (here in southeastern Pennsylvania) from months of rain and slow melting snows. Waiting until the Fall is much riskier – Success wise – because of the possibility of long periods of dry, rain-free weather. No rain means hard ground and limited penetration.
Last weekend I aerated my lawn; so I am now ready for some serious lawn treatments (currently pre-emergents for crabgrass) and a few patching jobs on some damaged areas.
So when will you be aerating?