Rough summation of the current rules at our lot:
Christmas duty defined as anything that impinges on Christmas day (midnight to midnight UK local time) itself, period, nothing either side..i.e. Christmas Eve and Boxing day are normal working days, as indeed are New Year's Eve and New Year's Day.....and we are not an LCC.
Any duty which involves you being away from base on Christmas Day attracts a number of Christmas points, which you bank over the years...if you have Christmas off in a particular year your banked points decay....Christmas points are in a separate "account" to the other points we get that are associated with leave/time off on other days of the year.
In rough terms the more Christmas points you have in the bank the less likely you are to be assigned a Christmas Duty...but there are no guarantees...
By choice I have worked the vast majority of the last 25 plus Christmases (Long Haul)...we so far at least have had a good staff travel agreement for getting family away on trips over Christmas ( which tends to mean there is no great shortage of volunteers for the Christmas work, on Long haul at least) and TBH being "out" avoids the which in-laws go where this year argument.
....( like many non aviators I don't think either sets of in-laws actually ever believed or understood why anyone would be working over Christmas, volunteers or not, but's that probably a subject for another thread)
When the kids were very young and travelling with them wasn't an option we simply did as parabellum described and re-rostered Christmas for another day in the period.