Me, whenever I feel like it. I have some Christmas-themed mashups on my MP3 player of choice, and sometimes I can't help but break them out once in a while (or if I play them accidentally).
That, or whenever BBC Three shows the Doctor Who Christmas specials during its repeats cycle.
: It's mindless now, clicking towards oblivion. ... How long, K-9?
: INSUFFICIENT DATA.
: Yeah, you never fucking know the answer when it's important.