Okay so we know that in Avatar the four elements and the four seasons correspond with each other and each Book is set in that time of year:
Winter = Water (Book 1)
Spring = Earth (Book 2)
Summer = Fire (Book 3)
Autumn = Air (Book 1)
Season one of Korra obviously starts out in early Autumn, and then in the finale Korra says: “I can’t believe a few months ago I was training for my firebending test…”, so we can assume it’s late Autumn or early Winter by the end of episode 12.
Book 2 is set six months after the finale - so you would think it’d be in the Summer, right?
BUT WAIT THERE’S A CATCH!
It’d be Summer in the NORTHERN hemisphere - in the SOUTH it’d be Winter!
And guess where Korra and the Krew are in the first episode - the SOUTHERN Water Tribe!
And what’s going on then? There seems to be a FESTIVAL of some sorts when a spirit arrives.
They must be celebrating the WINTER SOLSTICE! - a time when the barrier between the HUMAN WORLD and the SPIRIT WORLD is at its WEAKEST! (remember Hei Bei?)
That’s why the spirit was able to cross over and attack the village (and Korra)!
So I guess Book 2 will be set in both the Winter and Summer - let’s hope for lots of spirit world shenanigans ;)