I've recently done a little research for myself on the timing of CS's and how it can affect bubs... I'd recommend reading these two articles:

http://http://www.webmd.com/baby/news/20071211/elective-csection-38th-week-too-soon

Cesarean Birth and SIDS | January 2006 | RT: For Decision Makers in Respiratory Care

While the hospital might be saying that there are no other appointments available, I find that hard to believe... For some reason the medical profession likes to schedule CS's for 38 weeks. The only real explanation as to why that I've been able to get out of my OB is because leaving it later may mean that you go into labour and it becomes an emergency CS... None the less, I have told my OB in no uncertain terms that if I do decide to go with a CS there is no way I will be scheduling it until 40 weeks. At this stage he is ok with that...

Anyway, I guess the point I am trying to make is this - if you feel it is in your bubs best interest to wait until 40 weeks to schedule a CS there is nothing that the hospital can do. If you fail to show up for your booking, then they will have to squeeze you in on another date. And if you do go into labour naturally then they will most certainly be able to accommodate an emergency CS.

HTH