I've been trying to pay more attention to the season when I buy fruits and vegetables, but sometimes it is hard to figure out what is in season locally and what has just been shipped a long way (which says a lot about how disconnected the grocery stores are from the natural round). Some of the seasons are counter-intuitive: it seems obvious that lemons should be best in the summer, when we want lemonade, not in December (when they are). But even for the less confusing vegetables, it can be trying to keep track of what is ready when. I finally found one good way to think about it when I don't have the time or inclination to do research: Animal, Vegetable, Miracle, by Barbara Kingsolver, introduces the Vegetannual.
The vegetannual is a hypothetical plant that provides all plant foods, from roots to fruits, in sequence. First it puts out fresh leaves, then flowers, then seeds and fruits; in the fall, it plumps up the bigger fruits and stores sugars in its roots for winter. So the first vegetables in the spring will be greens - lettuce, kale, etc - herbs, and garlic scapes; followed by the early stems such as rhubarb, asparagus, and green onions. I think artichokes are a flower, which is why they are a spring vegetable, along with head lettuces, broccoli, and cauliflower. Mid-summer, the seeds and fruits will show up: peas, fava beans, green beans, raspberries and strawberries, summer squash, tomatoes, cucumbers. Late summer brings the bigger fruits, such as melons, peppers, and corn. Fall is the time for apples, winter squashes, pumpkins, and the tubers such as potato and yam.
The vegetannual has helped a bunch, but I still don't understand why citrus fruits, which seemingly should be ready in August, ripen in mid-winter.