Everything within the video games market is based on schedule. Games must be developed within a certain time window. They then put it up for a scheduled sale which falls on a day that makes sure their sales will be maxed. All announcements are also scheduled way in advance and fall into place on schedule.
The only problem with everything being scheduled is when the schedule changes it is extremely hard to make sure the news doesn’t get out. This is what happened with the dashboard leak of Gears of War 3. When Cliff Bleszinski got bumped on Jimmy Fallon for that pop singer kid that sounds like a girl (You know who I’m talking about). Cliff planned to announce the Gears of War 3 news on that show, and the Xbox Dashboard would follow suit. Plans change and the Xbox Dashboard kept following the schedule. The news was quickly removed and will most likely be put back when the actual announcement takes place.