From post #5 of the official Poly FAQ Thread.
http://disboards.com/showthread.php?t=2985839
You can also catch a movie on the beach between the Polynesian and the Grand Floridian. The movies vary by month and are shown on different nights of the week. Be sure to check your resort activity flyer upon check-in.
The time depends on when it gets dark. I'm going to guess around 7:30/8:00 in April.
And that info is now wrong. That area is now the GF DVC building. The Poly movies are shown on the Poly's main beach in front of the Volcano (Nanea) Pool. They start at about dusk.