You'll also want to try mitten crabs. They'll be in season.
你也要试一下大闸蟹，它们会正当季。

Mitten crab/Shanghai hairy crab
大闸蟹

in season/seasonal
（水果，蔬菜）当季

Seasonal fruits and veggies taste better.
当季的水果蔬菜更好吃。

Off-season
反季，淡季

It's the off-season right now. That's why there're not many tourists.
现在是淡季。所以没有很多游客。

I do a lot of off-season shopping and buy my summer clothes in the fall when they're on sale.
我经常反季淘货，在秋天打折的时候买夏天的衣服。