Spring or Fall?

Fall is usually the best time to aerate. The reason for this is that aeration usually exposes a lot of soil. Doing so in spring means exposing soil to warm season weed seeds like crabgrass. Crabgrass needs both heat and light to germinate. Once it does, it can quickly dominate a lawn with so much bare soil to work with. Crabgrass and other warm season weeds typically die off with the first frost. If you aerate in the fall, you may still get some warm season invaders, but the upcoming cool weather will take care of them.