I find that each sprint is different anyway, with varying amounts of
things distracting the team: support issues, courses, holidays,
sickness etc. So when planning the next sprint I look at how many days
were lost due to all this stuff and take a guess at how many will be
lost in the upcoming sprint too.
It means that your teams "true" velocity is how much you could acheive
if all that external stuff didn't get in the way. Henrik Kniberg
mentions this in his (freely downloadable!) book:
He calls this "Focus Factor".
So I would say that the Christmas period is just a time when there's
more than the usual amount of stuff distracting your team from the job