You're right. "Is being" wouldn't work here. "It is going to be this December" makes sense, but it could still be "It is this December." English is my native language, and this is giving me a little trouble, because it could be either answer depending on what question is being asked to yield this answer. If I had to choose, I would choose "It is this December" because it sounds like this could be the answer to a question such as "When is the concert?"