I don't even think that it'd be successfully neutral if the go-live date of stories was the 21st rather than the blindingly-obvious 25th.

I don't think that would make it entirely culturally neutral, no, but it would make it a whole lot more convenient! That way Christmas wouldn't interfere with Yuletide so much! It's kind of a weird situation for everyone, in a way: people who don't celebrate Xmas often feel uncomfortable because of the obviously Christmasy tone of the fic exchange, but on the other hand, I've heard some non-Christmas-celebrators say that they like having something fun to occupy all their majority-mandated free time on that particular day. Whereas to people whose families celebrate the holiday intensively like mine do, yes the Xmas connection is all very festive, but all the actual celebrating prevents me from reading any stories! I'm either staying with family or have family visiting for the entire period of Christmas to New Year, and there is much eye-rolling and bitching from certain family members if I spend too much quality time alone with my computer during that period.