According to an internal document floating around, Rivers of Light at Disney’s Animal Kingdom will make its very first performance on April 6th.
The April 6th showing will be for Animal Kingdom cast only. All other Walt Disney World cast members will have access to previews of the show on April 8th, 9th, 12th, and 13th.

There is some sort of “premium” annual pass holder and Disney Vacation Club member preview event being held on April 15th which will hold 4,000 guests, but we expect this to have an added fee (should be announced soon). Two normal AP and DVC preview nights are scheduled right after that for April 16th and 17th.
A press event premiere of the show will be held on April 19th and the show will of course open on April 22nd to all guests.
According to the document, showtimes once the show debuts will be at 8:30PM and 9:45PM nightly.