No effect at all.
It is not a secret meeting, everyone knows about it months ago.
The people who believe it has any effect on the price have already adjusted to it and people who don't know about it probably don't care about it at all.
There still can be announcements/launches that are not known yet....
If everything is known including content of the keynotes, etc then there would be no need for a conference.
It might have an effect but not necessarily a big one. We'll see...