This depends on what part of the world you live in. In Canada and the United States, look for them in July and August. In the UK, find them from the end of August into October. In Australia, November to May is the peak season for blackberries. Typically, you can find blackberries year-round in grocery stores, but keep in mind that when they are out of season or being shipped from far distances, the quality and taste won't be as good as in-season berries.