With us, you get a chance to book xmas/nyear 1 in 3 years, if successful (by no means guaranteed) you cannot bid for xmas leave for the next 2 years. Bizarrely, if you are rostered off over xmas by good fortune, you are also blocked for 2 years! That said, I have worked twice (voluntarily) over xmas plus one sby, in 19 years. So not all bad really.
In my previous, much smaller, airline, you had three lists of pilots who alternately had the most-to-least priority for leave requests. Annoyingly, even though there were no flights on xmas day, they would still roster you sby!
This is actually a worthwhile question for anyone considering joining a particular airline, since it certainly can cause a lot of heartache when you have a young family.