Cdog923

Why? Was the original date conflicting with another big movie?

There's no conflict; I'm assuming they moved it up for a global release; it was already being released the 27th in Europe. This way, they can both market it that way, and help avoid spoilers for Americans for a week. 

What CDog said. And it sounds as though there will be significant spoilers in Avengers 3: The Avengening (e.g. Kree/Skrull War--characters not who they actually said they were the whole time, characters dying, the location of the Soul Stone (I'm wagering it's on or part of the yellowish planet we've seen in the trailers). 

Good move by DisMarvel--not sure why they even bothered with different release dates in the past, unless it's a logistics thing.