Depends if you want to sell direct or not, 17 pigs us going to need a hell of a lot of customers to be found from scratch in time for Christmas (though I'm guessing you'll want to retain some pigs?). Have a search of the forum for threads about selling direct as there is a certain amount of red tape, especially once you get past selling to 'friends and family' stage.
How old are the pigs you have in mind and what breed? That'll have a bearing on if they will be ready to slaughter at the right time, also have in mind that some places won't do private kills in the run up to Christmas (though ours is a small operation that also does our cutting and I think this is more the reason they want to keep it to just their own work in December, it's completely understandable!).
For Christmas boxes you will almost certainly want sausages, ham and bacon so will need to make sure you have time to produce this as curing takes more time than picking the meat up jointed. We pay a flat fee of £60 kill and cut per pig which includes a couple of bags of sausages from the trimmings, any curing or vast amount of sausages costs more on top.
Not trying to be negative but thought worth pointing these things out, it's nearly October already so you'll need to get cracking but if you can get everything in order you and your customers should be enjoying some fantastic pork for Christmas!