Set out about 4-week transplants from April to October. The April planting will mature in mid-summer, the October planting the next spring. It is very important to keep the soil evenly moist and fertile at all times to avoid stunted plants and ensure optimum performance. A cool and moist summer is ideal for cabbage, producing big heads without bolting (going to bloom), but most Seattle summers will give fine crops when you plant the right varieties. Never expect every cabbage plant to actually produce a head. Depending on the weather, some varieties have a yield of only 50 % or less. Especially the over-wintering varieties tend to under-produce, so I recommend planting about twice as many plants as you want heads. Charmant is an early-maturing summer variety which I've found to produce well despite hot spells and irregular watering. Danish Ballhead requires all summer to mature and tends to become somewhat bitter if the late summer is hot, but the bitterness usually disappears after a few weeks in winter storage. Springtime is a good over-wintering cabbage variety.
Prepare your planting bed with plenty of organic fertilizer and give additional fertilizer once a month. Infrequent deep-watering in combination with late-summer hot spells tend to split the heads when they are about ready to pick, so try to water your cabbage patch more often than e.g. your tomatoes and potatoes, and provide some shading. Most P-patches are infested with the club root disease which attacks the roots of all cabbage-family plants, but I've found that keeping the soil fertility high will produce fine cabbages despite some disease on the roots. Screen out the cabbage maggots with Reemay or sawdust around the base of the plants. Even without screening, the maggots seldom kill more than about 30 % of the plants.