What Are Some Time-saving Tips For Preparing Christmas Dinner?

Prep veg/potatoes the night before (chill in fridge).

Make sure you leave the turkey out at room temperature the night before -- this is safe as long as it was kept well-chilled beforehand.  You want the turkey at room temperature before you start cooking; this will save hours and is what most estimates of cooking time assume.

Plan the meal to be ready half an hour before everyone will actually start being hungry.  Then if you run late you won't feel so stressed.

Alternatively, cook and prepare everything you can on Christmas eve to heat up on the day.  Then store in fridge or somewhere chilly and safe from pets/pests.  You can even do this in a half-way, such as parboiling (partly boiling) the veg and store in chilly place overnight.

If you aren't feeding that many so a turkey seems too much hassle, go for ham or a small roast instead.

Work out and write out a schedule of what to do when, allowing ten minutes more than you think you actually need for each step.