I think you could harvest it year around if you had the right growing conditions. In most climates, I've experienced, it does not grow much in the winter and could be covered with snow. In the hot part of the summer it can stop growing too. If you planted enough of it, you might be able to harvest something though.

I think of harvesting it more in the spring because it grows a lot faster then and it is one of the first things ready in the garden.