how far south do you have to live to have a year-round farmer’s market? I have only lived where you have them maybe June - September at the most.
You might want to define which country you’re talking about. 
I’m guessing your not from the West Coast of the US. I live in Northern California, and all the FMs I’m familiar with around here operate all year round.
Yep. Bay area, year 'round. Checking online, looks like Portland shuts down from mid-December to mid-March.
Old Strathcona Farmers Market in Edmonton Alberta is open year-round. I wouldn’t exactly call Edmonton “south”! 
New York City’s Union Square Greenmarket is open year-round.
Saint Louis’ Soulard Farmer’s Market is open year-round (thought not 7 days per week).
Nashville’s has an indoor section.
Fun to visit.
be damned, NYC doesn’t surprise me but Edmonton? places like Miami I just assumed.
I don’t mind hearing about different countries. 
so - if around San Francisco is year-round and Portland is not, wonder about the places in-between.
Eastern Market in Detroit is open year-round.
Though the Mpls. Farmers market is officially open from the last weekend in April through the end of October, there are sellers there every weekend through Christmas and every other Saturday through March. Then they start appearing every weekend. St. Paul offers stuff every Saturday during winter and also offers an indoor market until spring rolls around again.
Montreal - the Atwater Market and the Jean Talon Market (though there may be others as well).
for some of these Northern examples, I wonder how many of them only have gift-y type stuff & so forth in, say, December.
I mean, sure you can get home made jam but can you get fresh produce?
The farmer’s market in the (defunct and gentrified) Port of Tel Aviv is open year-round, as is Shuk Hacarmel, which is not so much a modern farmer’s market as it is a classic Middle Eastern *suk *that sells everything and anything, including fruits and vegetables better and cheaper than anything you’ll find in the supermarket.
It’s true that most of the NYC vendors this time of year sell cheese, wine, pickles/jams, eggs, fish, meat, etcetera, but there are still veggies available. There are root vegetables, stored squash and apples, and a variety of greens that can be forced all winter in a greenhouse, like kale.
When I lived in Dubuque, the farmer’s market did just have craft items, dried herbs and seedlings the first month or so that it was running, and we didn’t start seeing actual produce until later in the month (open May through October). It looked like quite a few of the local farmers spent their winter making wooden toys, cutting boards & the like in their wood shops.
Bangor Maine’s European Farmers Market is year-round.
The Flint, MI, farmer’s market is year around.
The local farmers aren’t, obviously, selling much locally-grown produce in January but there are vendors who sell imported produce year 'round, and you can buy dried locally grown spices and grains, meats, cheeses and plenty of other foods as well as gifty and crafty things.
I expect most cities have year-round farmers’ markets. They just have to move indoors come winter, is all.
Not only is the Rochester Public Market open year-round but a number of smaller local markets now have regular winter hours.
Calgary, Alberta has a year round farmer’s market, including fruits and veg.