When I was at National Lab, December was one of my favorite times to work. The lab would start to empty out after the first week as people took their vacations. It was especially common for senior people to be gone, since many of them had reached their maximum banked paid time off, and had "use it or lose it" days that they wanted to use. I really liked being able to work uninterrupted, get access to popular equipment, and carry out sensitive experiments without worrying too much about noise and/or contamination due to others working in the same area.
As a PhD student, there were advisors who were infamous for their heartless vacation time policies (like 2 weeks a year, not during December or summer) to try to maximize research productivity at naturally slow times for other academic responsibilities. I would never have joined such a group, since I know I need my downtime. It also strikes me as monstrously unfair for professors to use academic flexibility for themselves and deny it completely to their group members.
My own advisor was pretty cool about time off, and I follow a similar policy--I tell my group that they can have a"reasonable" amount of time off, where reasonable is in the 3-4 weeks a year range at their discretion, as long as they are making good progress on their projects. I encourage them to stay on campus as much as possible during the summer or to take their summer time off as a large chunk at the beginning or end (popular with students who need to travel far to visit family) rather than taking lots of long weekends, but have no policy (or comment) about December other than don't work in the lab alone.
Personally, I think travel flexibility is one of the best perks of the academic life. I can work from anywhere, since a lot of what I do is reading and writing, so while I rarely take really long stretches completely off, I do like to be able to travel a lot more than I did as a junior Federal employee with limited paid time off. I like to be able to extend this (free) perk to my group members, and I think it makes for a happier and more productive group in the long term.
Happy holidays to my readers, whatever you celebrate!