How likely do you think it would be to get an interview in the US when universities have their finals and right before they leave on Christmas break?
It's possible, but in my experiences as an applicant and as a search committee member, there tends to be a bit of a lull in the hiring process for most of the Thanksgiving - New Years' period. That said, there usually is a decent "round two" with searches January - March. So even if you're turning up naught, something could well happen for you in March or April provided you're considering places outside of the R1 and elite SLAC circuits.
DevGrad I'd say it depends on a few things. First, how badly they want to get the process done quickly, like if they start interviewing people in early December, they might want to finish it out, even if one candidate has to come in at that late semester time. Second, how important students are to the process/school. SLACs might want you to teach a class and/or meet with students. If there's no class for you to go to in order to teach and/or there aren't that many students around, then they can't really interview people during that time.
I'd say in contrast to what DocJ said, I had substantial experience going on interviews during that late November / early December time and/or doing phone interviews at that time and coming in to interview in January. There was definitely a lull, but it didn't start until mid- to late-December. If they're trying to offer you the position and be in touch with you about an offer around this time (i.e. if you interviewed in December), there might not even be a lull over Christmas break (except for Christmas Day itself and New Years Day.) Though it's hard for a department to get things together at that end-of-semester / holiday time, a lot of them are eager to get it wrapped up in order to get an offer out before the candidates are snagged by other institutions.
One thing that is very unlikely is to have some candidates interview in December before break and others in January after break, unless the ones who are invited later are only invited after the search fails with the December candidates (either the offers are turned down or everyone is unacceptable.)