Seasonal eating is usually presented as a discipline, with charts and rules. In practice it is simpler and more selfish than that: things in season are cheaper, need less done to them and taste of more, which makes them the easy choice rather than the virtuous one.
You can see the season without looking it up
Whatever is piled high, sold cheaply and looks good is in season. That is the whole method. A vegetable grown nearby and picked days ago sits in a big heap at a low price; one flown in sits in a small display at a high one. Price is the most honest signal in the shop. When something halves in price and improves at the same time, that is the season telling you what to cook.
Seasonal food needs less doing to it
A tomato in August wants salt and oil. A tomato in February needs roasting, sugar and an hour to become interesting. The cooking effort required is inversely proportional to how ripe and recently picked a thing is. That is the practical argument: in season you cook less and eat better, which is the opposite of how most food advice works.
Buy the same things for a month and vary the method
Seasons are long and vegetables are few, so the risk is boredom rather than scarcity. The answer is to change the method, not the ingredient: roast it, then shred it raw into a salad, then blend it into a soup, then fry it hard with chilli. The same cabbage can be four completely different dinners across a fortnight, and each of them costs almost nothing.
Preserve the glut in the simplest way available
When something is cheap and abundant, buy more than you need and put it away without ceremony. Roast and freeze tomatoes. Blanch and freeze beans. Cook and freeze a big pot of anything. This is where the savings live. Food bought at the bottom of the price curve and frozen is the same food in January at a third of the cost.
The exceptions worth knowing
Out of season, frozen almost always beats fresh, because it was picked at its peak rather than picked early to survive the journey. Tinned tomatoes beat fresh ones for most of the year. And the storage vegetables — onions, potatoes, carrots, cabbage, squash — are effectively in season for months because they were grown to keep. They are the backbone of winter cooking for that reason.
Buy what is cheap and piled high, do less to it, change the method rather than the ingredient, and freeze the excess when it is at its cheapest. That is seasonal cooking, and it requires no chart at all.