Fall is the best time to aerate a lawn because there is minimal weed pressure and the lawn's grass is in an active growing phase. Although some homeowners aerate their lawns in spring, it is not the proper time because aeration holes in the lawn allow weed seeds to germinate.
The seeds of crabgrass and other types of weeds are the first plants that germinate in spring. The aeration process stirs weed seeds up and creates an ideal environment that allows them to grow unchecked. Aerating the lawn in late spring is only beneficial if the lawn's thatch is over 1 inch tall, the lawn's turf is excessively thin or if the ground is compacted.