Providing aeration for your grass entails dealing with thatch -- which, along with compacted soil lies at the heart of the matter.
The build-up of thatch makes it difficult for your lawn to breathe. Lawn aeration performed in spring or fall helps control lawn thatch. The process of lawn aeration be as simple as poking holes in the soil here and there, but this would work only for superficial cases. For those in greater need of lawn aeration, this haphazard approach will not be sufficient: you will need to perform core aeration. You should also faithfully remove as much lawn thatch as you can in spring or fall by thatching/power raking your lawn.
Lawn aeration also breaks up compacted soil, allowing water and fertilizer to permeate into the roots. Grassy areas submitted to constant foot traffic require lawn aeration more frequently than do out-of-the-way areas. Regular lawn aeration should be preformed every spring and fall.
For our PNW lawns which are cool-season grasses. We recommend that the spring and fall seasons is better suited for core aeration due to the fact that the cool season grasses are in their active growing stage. This is the reason that core aeration and over-seeding go hand in hand.
I recommend spring aeration be done between February 15th and June 15th then fall aeration done between August 15th and November 15th annually. Summer aeration is recommended for those who water their lawns during this season and are looking for water retention and better penetration to get that water to the root zone. Summer aeration WILL help cut down on water cost to keep the lawn green during the heat.
The lawn aeration equipment we use will pull plugs, or "cores" (thus the name) of soil out of the ground, letting air, water and nutrients in. These plugs can be 1.5 -3 inches in depth. Such a plug should be pulled out of the lawn at about every 3 inches. The plug removal process is facilitated by watering the lawn the day before, but don't water to the point of muddying the soil.
Likewise, if your thatch problem is severe (1/2 inch thick or more), I recommend having the lawn de-thatched/power raked which can help alleviate the issues mentioned above.