The Governing Body of Jehovah's Witnesses are all about control of the lives & the information their members are exposed too.
As others have pointed out, if important announcement information were given out at the first weekend of DC's, then soon all the JW community would know of it well in advance of their respective DC's. In that respect the GB would have no control of the announcement or any twists, rumors, etc. of the new announcement.
The annual GB meeting will be the platform for any special announcements, or new light. With this the GB has full control of a captive audience.
The DC's will be business as usual. Same boring crap, guilt trips, the end is really close now, listen obey & be blessed, new literature releases, drama, do more to serve us, doesn't the new light really make sense, and oh yeah.....we need more money.