Growing up (the few years we had a garden -- my parents were not into gardening) September was the month in which we pulled everything out and the patch behind the garage became a boring and barren place until the following June. But September is a great time to sow a few seeds in the vegetable garden. It's very likely that you (like me) have been pulling out tomato plants that have ceased to be productive, or maybe you've just gotten tired of dealing with powdery mildew on your cucumber and squash vines. This doesn't mean that your garden is finished -- there are plenty of vegetables you can plant in September, no matter which region you garden in.
In my case, I'll be planting mesclun, turnips, beets, spinach, peas, and mache this month. Some of it will be planted in areas that recently held tomato plants and summer squash, so I'll be adding a good layer of compost before planting to increase the nutrients in the soil. I'll also start thinking about what I want to plant in my low tunnel for winter crops -- most likely, mache, lettuce, and spinach.
Of course, sometimes, you are ready for a break from gardening, and that's fine too. Some years, I'm gung-ho to keep gardening, and others, not so much. Even those years in which I'm feeling less-than-enthusiastic about gardening, I end up throwing a few seeds down. It's always nice to see something green and edible growing in my garden in October!