According to Senior Editor at The Verge Tom Warren, this year's Microsoft's E3 conference will held on Sunday June 10th at 1PM PT / 4PM ET. It's the third conference that has been official confirmed, after Bethesda's press conference that will be held on Sunday, June 10 at 6:30 p.m. PT/9:30 p.m. ET/2:30 a.m. GMT on June 11/11:30 a.m. AEST on June 11 with EA Play from June 9th to the 11th.
Last year, Microsoft used their annual E3 media briefing to announce that Project Scorpio is actually "the most powerful console ever" Xbox One X and released trailers for Metro Exodus, Crackdown 3, Forza 7, Assassin's Creed Origins, Sea of Thieves and Anthem. This year, we should expect to witness the return of the Fable series from the creators of Forza Horizon, new Gears of War and Halo.
Warren has also tweeted that "Microsoft is doing something different for E3 this year. I think that “different” element will start to make sense soon..." I guess it means that the conference will be held by the developers, just like Ubisoft's conference during E3 2017. Microsoft probably won't announce new hardware and maybe this time, Phil Spencer will talk more about games than teraflops...