Ok this is how it works at my outfit:
Definition christmas: 24th december 1800LT (homebase) until 26th december 1600LT
Definition new years: 31th december 1800LT until 1 jan 1600 LT
If you have ANY duty during that period, you are defined as “working” during christmas and/or new years eve.
If so, that means that the next year is guaranteed free of duty at home.
This has never caused any problem at all, many flights are cancelled anyway during those days. This has been in effect for decades at my company. Works great.
Only downturn: if you were working last year, don’t expect to be able to request anything during the holiday time slot.