Presumably at gamescom
Infinity Ward executive producer Mark Rubin has teased the big multiplayer reveal for Call of Duty: Ghosts as happening during the middle of August. "We are in our death march push to finish game," he said on Twitter. "Lots left to do."
With gamescom set to take place from August 21-25, that seems like a natural venue for such an unveiling. That said, Call of Duty is big enough that Infinity Ward could pick a day, any day, and dominate the news cycle ahead of the expo.
With Battlefield 4, DICE started off its promotional efforts by talking about the game's single-player component before demonstrating the multiplayer at E3, which is what the majority of us were eagerly waiting to see all along. In the case of Ghosts, the setup is similar -- to the point where what little information we have about the title's multiplayer has to do with, get this, a falling skyscraper. Some things never change.