In a surprise move, Janet Varney (the voice of Korra) announced a challenge this morning, through the Korra Nation Tumblr account: If her post was reblogged 10,000 times in 12 hours, we would get the hour-long season finale tonight on top of the hour-long episode we were getting today anyway.
Ten of those twelve hours were unnecessary, of course, as the post hit 10,000 reblogs in a mere two hours. And so the finale of Book 2: Spirits will be available to watch online tonight at midnight, Eastern Time.
The season had been gaining momentum since the extraordinary Beginnings, the hour-long episode that told the story of the first Avatar. It was simply one of the best stories we’d seen in the Avatar world and since then, Korra has matured and is facing a huge adversary. The stakes couldn’t be higher. And after weeks of building and setting everything up, we’ll get 4 episodes tonight to see how it plays out.

Sounds like her, anyway.