I never did figure out why Microsoft can go a day early.
They don't.
The exhibition hall opens when E3 starts on Tuesday.
It just so happens that Microsoft spends a jillion dollars putting on a giant show for the media in another building in the same city, while all the game journalists are in town, on Monday.
Its not a matter of "why they can" or "day early". Its a matter of MS having the belief that getting out in front of
E3 proper and setting the tone by being the first thing people see when they start checking coverage of E3 is worth the millions of dollars its costs to rent a stadium in LA.
Uhg, I got off a nightshift and am going to be staying up to see the press confrence, so its a good thing I don't work Monday nights.