All year round, but best from mid July to mid November.
Choose the best:
Larger bulbs can be tough, so select a medium-sized one that feels heavy for its dimensions. The leaves should be crisp-looking and intensely green. Avoid any bulbs that have soft spots or yellowing leaves.
Snip off the leaf stems, trim off the base and top, then use a potato peeler or sharp knife to peel it as if it's an apple. Then thinly slice, chunk or cut into wedges. If you're using slices in a salad, blanch them first.
Trim off the stems and keep in a perforated bag in the fridge - it will last up to two weeks.
To roast, steam the bulb for 5 minutes, then roast for 45 minutes. Steam (up to 12 minutes). Stir fry (up to 6 minutes). The leaves can be cooked like cabbage.
Details about Kohlrabi from here