Travelling over the Christmas Holiday - Planning

[trainJam]

Usually I avoid travelling or riding around in the period after the 25th December until after the New Year. However, I might consider getting out and exploring the railway in this period this year. There are some gaps in my understanding of planning, train timetabling and associated data.

National Rail or TOC websites advise that some timetables may be "amended" - what sort of changes are usually made?
As someone who uses walk-up tickets more than fixed-itinerary advances, having a timetable (e.g. pdf) is useful to refer to, even with journey planners - where are these amended timetables published?

How early are these amended timetables shown both on public journey planners as well as RealTimeTrains/OpenTrainTimes/live rail record/any system using similar data?

Are there likely to be more instances of cancelled trains caused by the "ten o'clock the night before" (can't remember the name) system?

What are your experiences of travelling in this period, and what is different?
Has this changed significantly post lockdown?

 

[30907]

Journey planners should be up to date, and some TOCs certainly have PDFs.

Impossible to give a general overview of Christmas week services - LNER/Avanti seem to be running a full timetable, for example, whereas some SE routes seem to be on a Saturday timetable (but there is relatively difference between Sat and M-F since covid). There's significant engineering work eg at Victoria and Waterloo.

Traditional commuting times are likely to be quiet, but other times busier than usual.