Depends on the variety - sow from Feb to May, then transplant to permanent position in June. Early varieties will start to crop in Nov or even earlier, hardy types can go on cropping until March or April.
They grow very easily from seed. Be sure to plant them out into very firm ground ie not freshly dug, as that will make them blow. For tight buttons the ground must be very firm. It is also a good idea to stake the plants if your area is at all windy as they lodge very readily. One good trick is to plant them out in a hole about 6" below the surrounding level, then as they grow earth them up - this gives plenty of strong deep roots.
Cabbage root fly, mealy bugs, cabbage white caterpillars, slugs, mice, pigeons and rabbits will all have a go at your crop.