At the Farmers' Market: Squash Blossoms
Edible blossoms form on all squashes, both summer and winter, so they are typically available fresh from April until well into December.
Plants produce both male and female flowers; female flowers form on the end of a squash and male flowers do not. Both are open together for only one morning, meaning that bees or humans must pollinate the female blossoms in order for there to be squash. Typical squash plants produce three times as many male flowers as female to increase the chance of fruiting, which means that there will typically be more male flowers for sale than female.
The most common preparation of squash blossoms is to stuff them with seasoned soft cheese, close them, dip them into a light batter, and deep fry them until golden brown. This preparation requires the freshest male blossoms; fresh for ease of stuffing the flower, and male so that the attached vine stem can be used as a handle while frying.
Yesterday's recipe for quesadillas is agnostic as to the flower's gender. Female blossoms with tiny squashes attached are often sold as farmers try to thin the impending glut of squash. The tiny squash is perfectly edible and is, in fact, softer and sweeter than fully mature squash. (Bonus fact: when I lived in the Midwest, the only time I ever locked my car was in August, because well-meaning but desperate neighbors would leave bags of giant, woody zucchini in my front seat if I didn't.)
Other suggestions for the flowers: shred them and toss them into salads for a slightly nutty taste and a splash of color; stir them into risotti or pilafs; sautéed with shallot until wilted, then piled on grilled slices of good bread; tossed into clear zucchini soup.
If you are going to stuff the blossoms, buy them fully opened (the specimens from Sweredoski Farms at the Irvine farmers' market are the finest quality I've ever seen); if you are going to cook them down or shred them, then the piles of slightly wilted blossoms will do you fine. Avoid blossoms that have turned brown, and as with any leaves, slimy is never a good thing. Wrapped like herbs in a damp paper towel and stored in a zip-top bag in your refrigerator's crisper, you may get four or five days out of them, but for best results use them the day you buy them.