Lentz's will help you determine the best time to aerate your lawn to improve its overall health.
Benefits of Core Aeration
Loosens compacted soil and increases the availability of water and nutrients.
Enhances oxygen levels in the soil, stimulating root growth and enhancing the activity of thatch-decomposing organisms.
While removing cores of soil, the spoons or tines also sever roots, rhizomes and stolons. Grass plants are stimulated to produce new shoots and roots that "fill up" the holes in the lawn and increase the density of the turf.
Reduces water runoff.
Increases the lawn's drought tolerance and improves its overall health.
The type of grass will determine whether to aerify in the fall or in the summer. Lawns composed of cool-season grasses such as Kentucky bluegrass and tall fescue are best aerified in the fall, when there is less heat stress and danger of invasion by weedy annuals. Allow at least four weeks of good growing weather to help the plants recover. Warm-season grasses such as zoysiagrass, centipedegrass, carpetgrass, St. Augustinegrass and bermudagrass, on the other hand, are best aerified in late spring and summer, when they are actively growing. With either type of grass, choose a day when temperatures are mild and soil is moderately moist, which makes the soil easier to penetrate. Avoid aerifying a wet soil, as it is messy and leads to further compaction of the soil as well. If the soil sticks to your shoes or if the core sample you take sticks to your probe, you should wait until it dries out some before starting the job.
Aerification of home lawns corrects soil problems but generally is not a routine practice. The best answer to the question, "How often should I aerify?" is, "As often as needed." One way to determine if aeration is needed is by scouting the lawn. Take a screwdriver and probe the soil. If the screwdriver penetrates the soil with little resistance, then you probably don't need to aerify. If it is difficult to penetrate the soil with the screwdriver, then you may need to aerify. Make sure the soil is moist when testing the areas since dry soil can also be more difficult to penetrate.
Read more about lawn aeration here:
http://www.clemson.edu/extension/hgic/plants/landscape/lawns/hgic1200.html