Not meaning to double post, but here is some interesting info
Dreamwork's Monsters vs. Aliens is a 3D animated film that was set to open May 23rd
Not too long after the word got out that it, and Avatar, another 3D film, were facing head to head on May 23rd, 09, people starting speculating which would change its date first, and many were thinking them facing off wasn't good, simply because 3D screens around the world would have to choose to play either Avatar or MVA come the 23rd, and them both being pretty prestigous productions, many theatre chains didn't want to have to make that descision.
But time went by, aand eventually, Dreamkworks changed the release of MVA, bumping it up to May 15th, thye conceeded, and changed their date from the big memorial day weekend.
However, most thought with just a weeks difference between the releases, that the 3D screens around the world would have to still be divided, and that niether big 3D film would get to get the full glory of a wide 3D release.
Fox isn't dumb, they know Avatar is going to be huge, and never once displayed concern, or changed from their memorial day weekend date
so as most predicted, Dreamworks ended up changing their date for MVA two months ahead of Avatar's all the way to March 27th, one of the execs claiming that they didn't want to divide 3D screens between two film when they were supposed to be at their biggest.
so now, Avatar has Memorial day weekend all to itself, and originally, May 1st was going to be the release of the 3rd Chronicles of Narnia film, and some thought that would be Avatar's biggest contender, despite the two films coming out 22 days away.
But that too has been pushed back, for production reasons, to May 7th, 2010
and so now, the only film even listed to come out in May 09 is G-Force, some family adventure movie, coming out on the 1st, so again, it won't really effect Avatar much.
so basically, as of now, Avatar is going to have memorial day weekend all to itself, so it will get put in the maximum number of screens, conventional and 3D, its going to be great.