Unknown as of end of 2014, but probably Legends. (UPDATE: As per Richard's later answer, it's most certainly not canon as of 2015)
So far it hasn't been listed among "these are New Canon" items, but also not listed among items clearly going into Legends canon.
Unless and until someone from DisneyLucasFilm clarifies that, we can speculate that it's Legends (seems like a safe bet) but we just don't know for sure.
Wookiepedia lists it under "Legends" list but I wasn't able to find a cite, from either Chee or Heddle.
No canon level for the whole thing overall
It's NOT a movie (G-canon) or post-movie TV series (T-canon).
Bits and pieces range from C-canon (confirmed in other works) or S-Canon (Can be contradicted by C-canon level works in the future).
From Wiki: Canonicity of the special
The Star Wars Holiday Special is technically in the Star Wars canon, which means that the events depicted are part of the greater continuity that includes the other films, novels, comic books, video games, etc. Generally, it falls in the C-Canon in the overall Star Wars continuity.
According to Leland Chee, the keeper of The Holocron, an internal Star Wars continuity database at Lucasfilm:
contains at least 28 individual entries relating to elements of the holiday special, most elements from the holiday special are definitely considered canon; however, there are specific rules as to what is what.
First off, any element from the holiday special that is referenced in another work is considered C-Canon (such as Life Day, Chewbacca's family, etc.).
Any element from the holiday special that is not referenced in other works is considered S-Canon, which means that it is canon, and that it "happened," but its canonicity is not set in stone.
The only element from the holiday special where the canonicity was disputed was reused footage of Chief Bast, a character who was killed during the destruction of the Death Star from the first film. Despite being portrayed by the same actor, he is intended to be a different character.