According to Bethesda's executive producer and Game Director for the Elder Scrolls V: Skyrim, Todd Howard, the first expansion for the universally hailed RPG will be coming onJune 26. He said that with a qualification however, adding "as long as nothing goes wrong."
So while there's a chance we wont be seeing it arrive at exactly that date, it might be a good idea to clear out that entire week anyway. The first expansion will be called Dawnguard and casts players as defenders (or willing participants) against a tide of vampire-like creatures who wish to essentially kill the sun.
Apparently, this can have consequences for your game long-term, as players who destroy the sun may be plunged into perpetual darkness (as-in their save game is irrevocably immersed in night.)
This first DLC will be coming to the Xbox 360 (whenever it happens to launch.) We'll keep our eyes peeled for more Dawnguard info as E3 rages on.