Using まで together with a negative connotation 帰らないつもりです in a sentence


I am referring Minna no Nihongo 2, Chapter 31 Mondai section.

The sentence pair is:

> Q:夏休みに国へかえりますか。  
>  A:いいえ、クリスマスまで帰らないつもりです。

It would be great if you can tell me why the answer is 帰らないつもりです instead of
帰るつもりです? Thank you very much!




  * 帰らないつもりです - I do not have the intention of returning.
  * 帰るつもりです - I have the intention of returning.

The conversation goes:

> Q: Are you going to return home for the summer holidays?
>
> A: No, I don't plan to return until Christmas. (hence the use of negative)

