Thatch is a layer that develops between the soil's surface and the grass. If the thatch layer is deep, the lawn will feel spongy. Use a spade to cut into the turf to determine the depth of the thatch. Dethatching is something I recommend doing only if the thatch is 3/4 inch or more thick, not as a routine practice. Core aeration, however, can be done at least annually. Not only does it loosen soil, it helps control thatch. If thatch is really thick and has gotten out of hand, aeration won't solve the problem. That's when you bring in a dethatcher (also called a power rake or vertical mower). But if thatch is more or less under control, it should stay that way with regular aeration. Spring and fall are both good times to aerate cool-season grasses.