Oysters are sold year-round, but they're firmest and sweetest from November to May, says Jon Rowley, a seafood consultant from Seattle (summer is spawning time, when oysters are softer and not as sweet).
Oysters come from all over the country, "anywhere the ocean is clean and cold," says Rowley. Kumamoto, Olympia, Belon, and Malpeque are just a few varieties you may run across. At the market, look for oysters that feel heavy in the hand, with shells that are shut tight—a good sign that there's a fresh, juicy oyster inside. Don't eat any open oysters, even if they shut after being nudged, cautions Rowley. While oysters harvested the preceding day are ideal, those culled up to a week before are fine. If in doubt, ask your fish merchant to show you the shipping tag; by law, it must be marked with the harvest date and affixed to the shipping container.
Once you hustle your oysters home, don't put them in ice, but do stash them in the refrigerator. Arrange each oyster so that its convex side faces down (the more deeply cupped half holds the flesh and liquor) and drape a wet towel over them so the shells don't dry out. A fresh oyster's incomparable silky-firm texture and minerally sweetness are best savored unadorned—or at the very most, with a squeeze of lemon. For good shucking technique, watch the video Shuck Oysters Like a Pro.