The Dota 2 page has been updated with a blog post that talks about the game's future changes and the Frostivus event. Namely that Frostivus won't be happening this year, because they're working on big engine changes.
The "major improvement to Dota 2’s engine" is due to be released in the first half of 2015. And the devs aren't bringing the Frostivus or Diretide events because the engine changes would required them to be rebuilt anyway. In short, it's not worth the time wasted, when they could just concentrate on the engine update.
The blog update says:
"With Diretide this year, we decided that it didn’t make sense to build another one of those resource-intensive game modes, for two reasons. The first was that the mode would quickly become obsolete when the engine changes arrive, and the second was that building it would have taken resources away from the work that needed to be done to ship the engine change itself. Now that Frostivus is on the horizon, we find ourselves facing a similar choice and, after some thought, we believe that once again the right choice is to not develop a Frostivus game mode."
The devs have said that this doesn't mean nothing is happening before the major engine update - they're working on the New Bloom Fesitival event which is due in February right now.
Speaking of New Bloom, a further update on the website asks for artists to create new items based around the Chinese New Year theme, just like last year.
Any submissions you want to make must adhere to a few rules, so make sure you read the update if you're thinking of submitting. Entries must be made by 19th January 2015.