I don’t have any experience in this, but you should apply now, stating in your CV that you wish to start after christmas. I think that should be alright? That should get your CV in early, and make it fit in with your studies.
Let me just say from my experience last year… I graduated in May 07 and it took me FOREVER to find a job! Apply Apply Apply! Doesn’t matter if you can’t start asap, just start applying because you never know how long it will take to find the right job, or one at all.
I would just let them know the exact date you would be available to work.
It never hurts to apply for jobs at any time. Even if you already have a job. If anything, it shows potential employers that you’re eager to work for them, that you’re driven, and that you’re focused on getting a job.
If you’re worried about being hired, just let them know that right now you’re devoted to your studies and you simply cannot start until you’re finished with school. Call them periodically to let them know you’re still interested. Even if the position fills up, the hiring manager might even know of another company that needs your skills. Talk with people. Let them know you exist, that you want to work, and that you’re motivated to get the job.