Well, they got us again. Frostivus has been cancelled this year because some ghost king is returning from somewhere.
The return of a wraith king is a pretty big deal (one would think so at least) so you can expect a lot of interesting features added to the game for this special event.
According to their Wraith-Night mini site players will be able to participate in daily events of varying difficulty and gain a bonus when the invite new friends to the game.
In addition players will be able to purchase a new playable character. The Legion Commander will be available today for purchase from the Dota 2 store.