The deadline is 15 January, and even though my school has made us work on our applications online for weeks now and we spent weeks working on personal statements before that, I suspect most people in my year will send them off in December or January. I'd say it's best to get it done as early as you can. Take your time over it and make sure it's as good as it can be, but don't leave it until the last minute either. Aim to get most, if not all, of your application done over the summer and then you'll just need your teachers to write your reference when you go back.