Fall is the time for aeration and liming. Most soils in the Willamette Valley tend to become acidic over the years. Your lawn likes to have a pH around 6.5. When the pH drops below this lawns tend to struggle and become less resistant to diseases. Also Moss that starts to take over your lawn in the fall and winter thrives at the lower pH levels. Putting down lime as the fall rains start to move in helps to raise the pH. pH testers are available at most Garden Center to find out the pH of your lawn.
Aeration of lawns has multiple benefits. The main benefit is it will allow more oxygen into soil and loosens up compacted soil on the surface. A side benefit from loosening up the surface soil is better drainage at the surface so less water runs off. Aeration should be done at least once a year or twice for high use lawns or turf maintained with large equipment. Spreading lime after aeration is best as it allows the lime to get directly into the soil.