I would also like to thank everybody who managed to work thanksgiving this year, because it meant that a lot of people got the time to spend with their families.
With regards to how shifts are allocated, from my experience people are given the chance to offer to work, but if nobody wants to do it (or agrees to do it) they will try to do it randomly so that everyone has the same chance of getting a shift. You will usually get paid more on a holiday, and depending on what job you're doing on the day you may have one or two fun things to do at the station! We always had a really good dinner for all of those who were around and not out at the time.