I think they chose that date because there isn't really anything significant (at least in potential blockbusters) that is out on that weekend. April 12 is when "Titanic 3D" is coming out as well as Luc Besson's "Lockout" (which could be hit or miss).
Remember the year we're talking about here too. Next year is pretty much going to be "The Year of the Blockbusters". Once summer hits all the way through to December there is a major movie coming out. They want to at least try to make a few million on this movie. I think April was pretty smart after thinking about it.
Besides which Joss just co-wrote the script with Drew Goddard, Drew directed the film.I don't predict a high box office intake for it. Chris Hemsworth is the star of the film so with his status now after "Thor" and the pre-hype of "Avengers" it's gonna be interesting to see how this does.