The Legend of Korra, Season 2

Korra:
So once we get to the South pole, then what happens?

Unalaq:
You will open an ancient spirit portal.

Korra:
I'm sorry. What now?

Unalaq:
At the South pole, there is a portal that connects our world to the spirit world. But it has long been closed.

Korra:
And that's why the evil spirits are attacking?

Unalaq:
There are no evil spirits. There is light and dark in them all. But when they're unbalanced, the darkness takes over. If you can open the portal in time, balance will be restored.

Korra:
What do you mean "in time"?

Unalaq:
There's a reason the glacier spirits festival ends on the winter solstice. That's when the spirit world and the physical world are close together. Only then can the Avatar open the portal.

Korra:
The winter solstice is tomorrow.

Unalaq:
Exactly. And we can't afford to wait another year.