Elementary, our dear Shmoopers. This one's set in the winter.
Ah, but not so fast. Sure, there are spruces and frost and boughs and pine trees and snow and January. But the wintriness of the imagery of this poem is really not at all what this poem is about. Winter is more an occasion than a setting, giving the speaker the opportunity to talk about what's really on his mind (of winter).
Much as with the "Speaker," the setting here takes a back seat to the philosophy being discussed. It's not so much winter that's important, but how we perceive it. If we're being our usual selves, we see it as bitterly cold and miserable. But if we're the snow man, we can see it for what it truly is, which is nothing at all.