I believe it is every Friday night. We were there for the 4th of July and I really wasn't expecting a movie with the fireworks and everything, but they still showed it. It was "Independence Day" I figure they probably show only movies that came from Universal, but I could be wrong.
They did pause the movie during the firework show. Watching the fireworks from the pool was so cool. I would much rather be there than in all that craziness at the parks and CityWalk!