Where does Apple announce upcoming press events? Most tech news web sites publish stories a few days before upcoming Apple hardware/software announcment press events.

How do they know that there will be a press event?
I couldn't find that information on Apples web site.
Information about an invitation-only press event isn't posted to the Apple website in advance  Those who are invited to the press event receive email invitations; those who aren't invited just report that there is a press event because they've seen the reports elsewhere.  The announcements that are made at the invitation-only press event will be posted in a press release after the event.

Apple rarely announces hardware or software outside of invitation-only press events.  One exception is their annual Worldwide Developer Conference, which starts off with a keynote address that is attended both by the developers who paid for the conference, as well as invited members of the press.  This keynote address includes announcements of new hardware, software, and operating systems.  It is the only part of WWDC which is publicly available and not covered by a non-disclosure agreement.  

Apple will occasionally simply update a product with no event at all, such as the recent iPad update.  In this case, they simply drop a press release and let it make its way out into the world.