If people knew that in one week it was guaranteed that the price would double, it stands to reason that they would buy now.  Of course that would bring the price up until it was doubled, and people would stop buying.

But then we would all know that that was what would happen, so a week before that we'd buy.  Oh... except then the doubling would happen two weeks before the guaranteed double date.

So a week before that... etc, etc.

If an event is known to happen in the future, then the market will ensure that the effect on its price then is included in the price now.

There will be no doubling of price in December; that event is already priced in.