Every spring we get the same call: the lawn was fine in August, and now it's half moss. Homeowners usually reach for moss killer, and it works for about three weeks. Then the moss comes back, because nothing about the conditions changed.
Moss wins in Western Washington for three reasons. The soil is compacted, so grass roots can't push through it. The soil is acidic, because nine months of rain leaches out calcium and pushes the pH down. And most yards have shade and poor drainage, which moss loves and grass tolerates badly.
The fix has to hit all three. Dethatching pulls the moss and the spongy dead layer out of the lawn. Core aeration opens the soil so air, water, and roots can move. Lime brings the pH back toward neutral, and a fall or spring fertilizer application gives the grass what it needs to fill in before the moss can.
Do it in early spring or September, when the ground is workable and the grass is actively growing. Overseed while the soil is open from aeration, keep it watered for three weeks, and mow high. The lawns that stay green through February are the ones that got this treatment, not the ones that got sprayed.