I am cooking for 15 this christmas so I know where you are coming from
Tip 1 - Don't panic! Sounds silly but it will help to keep a cool head
Tip 2 - Plan ahead, write down a timelined action plan - i.e. 10:00 put in roast potatoes etc.
Tip 3 - Recruit helpers - there's no reason why you cannot ask for help - if you have family coming over, ask them to bring something - e.g. Ask gran to bring her famous stuffing - it flatters the guest and eases the burden on you. LIkewise you can ask for help in the kitchen if you need an extra pair of hands.
Tip 4 - Prepare veg on the previous night - spuds, carrots, parsnips and sprouts can all be peeled or prepapred and soaked in cold water overnight without trouble
Tip 5 - Cook turkey early and serve it cold - this is perfectly acceptable (provided the plates and gravy are hot) - the turkey will probably take up your entire oven - better to get it out of the way - it carves better cold anyway.
Tip 6 - leave veg til last - roast potatoes and parsnitps can be cooked and kept warm but don't do the same to carrots, peas and sprouts - they tend to go manky.
Tip 7 - don't be a microwave snob, most veg cooks just as well in the microwave provided it is covered and has a splash of water with it.
Tip 8 - Warm your plates and serving dishes, this will ensure that food is served hot - even the hottest dish will cool instantly if the bowl or plate it is served on is cold.
Don't overcook your veg, it will keep on cooking whilst hot so if anything, get it out of the pan before it is fully cooked as you can alwaya put it back in - you can't un-cook veg - timings depend on quantities so just keep checking - it should be the last food to be cooked so timing shouldn't be as much of an issue
Remember to enjoy your day as much as everyone else - nobody minds waiting an extra half hour for a good meal - good luck!