I’ll keep an eye out for them! I have found them here in the past, but they’re rather rare here.
I don’t think it used to be a while ago. But we used pumpkin as a supplement in one of our former dog’s diet, and we could buy canned pumpkin year round. We did have to stock up before the holidays, because they would often sell out then.
That’s one I thought of. The oblong pinched plums (prune, Italian, Empress) have a narrow window but apple-shaped black or red are always around.
Fiddleheads aren’t worth the fuss but they scream spring. I’ve seen but haven’t yet bought/tried whole fresh sunflowers in the fall.
Corn, pomegranates, lychees, chayote are easy to find year round.
Lefse and lutefisk.
Well, they’re “standard” in parts of Minnesota.
I think that’s true for many produce items where the local ones are only available for a limited time but the imported ones available at other times of the year. I like peaches and in the summer, I get excellent ones grown in the United States. In the winter, peaches are available from Chile and elsewhere but are usually disappointing.
We give our dog plain canned pumpkin along with his kibble. A can lasts about six days, so every year we buy 60 cans between Thanksgiving and Christmas. After that, it’s either limited to one space on the lowest supermarket shelf at double the in-season price, or it completely vanishes. Pumpkin pie filling is available year-round, but it has too much sweetening and spices for dogs.
Cranberry sauce is available year round, but you often have to hunt for it and I suspect it’s not carried in a lot of stores.
Eggnog is typically only available for a time in December.
Fresh cherries.
You can get almost any other fruit year-round these days but fresh cherries are still seasonal.
Of course, you can get frozen cherries (or most anything) any time.
Fresh cherries are one of the “stone fruits” like peaches, plums and nectarines that are seasonal.
Just curious, where do you live that you can find fresh lychees in the winter?
Pomegranates and persimmons are one of the autumnal seasonal joys. They do not keep, they travel poorly. Like apricots, in July. Here in New England, local apples varieties start appearing in September but by January they are gone. My co-op has about twenty varieties of apples right now.
And then cranberries. Available frozen perhaps, but fresh only in fall and early winter. Ditto chestnuts.
I visited two groceries near Chicago tonight and confirmed lychees as well as the following:
Chestnuts, longans, rambutans, passionfruit, figs, mamey sapote, mangosteens.
We done the passionfruit! 
Is the old saying true - oysters “r” in season? (Pretty big season)
Fresh: table grapes
Preserves:fruit mince
Pawpaws! A friend has trees and gave me some. They aren’t sold in stores because they need to be picked ripe then eaten within a few days.
But they sure are cool. They are the largest edible fruit native to North America. They taste like a combination of banana, pineapple, and mango.
I’m currently stratifying some of their seeds. Assuming I get them to germinate, I’ll be planting them around our house and they should have fruit a few years after I’m gone.
It depends on where you’re looking. Almost anything, you can get somewhere, but maybe not from your usual grocery store. Aldi, in particular, considers a lot of things to be seasonal: For instance, most baking supplies can only be found there in the winter.
Interesting, I don’t see much stone fruit in Toronto in the winter. (Chestnuts are pretty common in the winter, of course.)
The Asian markets have them whenever they can get them but they definitely have a season (or two). Right now it is the end of the rambutans.
Red Haven peaches. Only available for a short time in NE Ohio. I always get them right from the farm. The ones in the store are seldom as good.