Definitely write a list of the meals you'll be having & then write a shopping list accordingly. Don't forget large foil for the turkey, goose fat, brandy sauce, cranberry etc. Maybe extra milk that you can freeze.
Prepare as much as you can on Christmas eve (eg peel veg & leave in water somewhere cold - garage or shed or car boot - just don't forget & go to pick someone up the next day with it still in there! 
Buy a Christmas magazine like Good Housekeeping or Sainsburys which will give you timings to follow.
Prepare food & freeze if freezer big enough. Not just stuff for the day but Christmas eve meal, boxing day food (freeze a curry sauce & chuck in some turkey, or freeze a chilli)
Don't cook too much. Maybe few m&s canapés & a glass of bubbly rather than a starter.
Maybe find out if the inlaws have anything they like to do at Christmas & try to incorporate & creep make them feel very welcome.
Take any offers of help (if they're forthcoming!)
Have a very large bottle of your fave tipple & if all else fails, get sloshed!
Finally, the shops are only shut for one day now so if you forget it, don't worry & if you run out, get it the next day! & enjoy 