I don't think SAs know until about a week before. If you are thinking of getting a bag, I would recommend getting it now. If you want until January, the price may increase and on top of that... it is "post-Christmas" inventory, and all the good stuff is gone.
Last year, I bought a kisslock in Nov, 2011 for $1395.00. Then, in February, the same black lamb kisslock increases to $1500.00 (I had a problem with my kisslock and had to exchange it which is why I knew the price increased). Then in May, I see a yellow kisslock that I like and buy it for $1900.00.
Totally insane. But... obviously not that insane as I still bought the darn bag. So.. Chanel knows what they are doing... demand for luxury items will always be there.
If you can get the bag now, I would definitely do so before the Christmas mad rush.