Look up "food seasonality charts" in google. It depends entirely on your location, though. Certain foodstuffs grow certain times of the year, in certain climates and conditions. Some are close by, others not so much.
I have a meat purveyor that sells veal only around August. Although animals don't necessarily have "seasons (but they actually do in a different sense)" it depends on when they breed or rear their livestock. Fish can also fall under "seasonality" if feeding cycles/ spawning/ salmon runs, etc occur during certain times of the year, as eastshores stated. Oceans have cycles that benefit some fish over others during they year providing abundance, especially in colder waters. Although IMO buying fish has more to do with availability and shipping, farm grown or wild, and which endangered species should be avoided - if that's something you care about.
Ask your farmers market vendors where they get their product. Get to know them; become their friend. Sometimes I have guys and gals throw in a few extra peaches or cut breaks on price. They will also generally be honest about where they get their product. One vendor admitted that the apples he had in October were from the year before, or the year before that, when I asked. Are the eggs washed? Where is your farm? Are you direct from farm? I also live in NYC. Some of us can get kinda pissed off if we are being cheated and will shut the place down faster than a rat scurrying away from the oncoming 6 train.
Also, I have one vendor that occasionally has great looking tomatoes in May or June. They are hothouse (greenhouse), but still grown by the farm under similar conditions - i.e. seed, fertilizer, pesticides etc. Most of the vendors at my farmers market come from the tri--state area, and are probably about 3-4 hours drive away. Some farther, like cheese and meat/ cured meat purveyors.
In the end, it has everything to do with a few things. 1. perceived quality of the produce/ product, and if it's GMO or organic or simply grown the way it was grown 200 years ago. Then there are hormones, antibiotics, and growth agents and animal cruelty issues to deal with. Also, I don't care that I'm paying $4.00/LB for heirloom tomatoes because : 2. Feed your local economy instead of corporate giants that skip on paying their fair share of taxes and screw over the environment for profit. Since it is a corporation, small things like you paying for their waste times millions of customers equals big revenue for them. Meanwhile, you're throwing away string beans that are rotted at the bottom of the package because they combined the good with the bad and you cannot select each one. 3. Shipping asparagus in to the US from Peru costs money and leaves a carbon footprint. Same with fruit from California. It's wasteful for a bit of convenience, but that's my personal philosophy (I do give in every so often, though). We've been culturally engrained to believe we can have whatever we want, whenever we want, no matter what the cost. And if it's cheap, even better. This ignores a host of other issues. I eat tons of asparagus in the spring/ early summer - when it's "in season" and avoid it the rest of the year. It makes for delightful spring months and I look forward to it each year. Same with August for peaches, September for tomatoes, and winter months for root veggies. This approach is entirely up to you, though. And then of course, it depends on what is available to you and what your budget is.