So when should I actually plant fall vegetables?

There's no dearth of advice about growing fall vegetables, but it's definitely a matter of figuring out your particular weather patterns, growing benefits and challenges, and thinking creatively about what to try (and sorting out what you read, and the location of the person who wrote it).

Our Southeastern U.S. Zone 7 (determined solely by frost dates) is quite different than Zone 7 in coastal Oregon, for example. And, different species need different lengths of time to mature before frost, and are affected by shortening days and cooler weather depending on the species (and cultivars), too.

a red-stemmed chicory looking unfazed by heat and drought

And, the recommendations of 'plant in mid-summer' or 'plant in late summer' are not terribly helpful when it's still 95° in the late afternoon. As a beginning vegetable gardener some years ago, I had the (very mistaken) notion that fall vegetables were planted in the fall.

In hot summer soils, trying to germinate seeds of cool-season vegetables such as carrots and lettuce often requires supplemental help in late summer (shading, damp straw, burlap, covering with boards). I've sprinkled ice cubes on the lettuce flats to help cool down the potting mix, following advice from an experienced local gardener. Carrot seeds are notoriously slow to germinate, in any case, as are some of the other species in the carrot family (parsnips and parsley come to mind). Wild species quite often have these stretched-out germination behaviors, and/or complex dormancy, partially as a response to optimize survival of seedlings.

An old way to germinate seeds that need cooler temperatures or that require perfect moisture balance is simply to start them indoors. Layering seeds in wet paper towels enclosed in a covered container provides a head start to triggering germination; the trick is to monitor them closely so at the first sign of young roots, you can transfer the sprouted seeds to prepared beds (before the root hairs attach to the paper towels). I suppose you could use a shallow layer of damp vermiculite or potting mix, too, but I haven't tried that.

What I've found most helpful are market-gardening publications (extension offices generally have good ones; I just printed out a succession planting chart that conveniently included space for your own notations about what you planted and when) and books (like Four-season Harvest by Elliot Coleman) that provide detailed information on different crops connected with frost dates, days to maturity, and recommendations for specific crops.

But there's always quite a bit of leeway, depending on how changeable one's fall weather patterns are, how late the first frost is in a given year, and climate change is now in the mix.

But, that's what makes it so fun to be a gardener.