Here is where I got the info. This is the July Activities Calendar for the Fort.
The Sing-a-Long is at 8pm. They light the campfire at 7:30pm with the movie at 8:45pm. There are "campfire activities" each day (different activity for each day of the week) that start at 7pm.
Looking at schedules I still have in my folders from recent years, the "summer" schedule was the Singalong started at 8pm and the movie 40/45 minutes later. The "winter" schedule (starting the month we turned the clocks back - November) shows the Singalong starting at 7pm and movie 40/45 minutes later.
No guarantee, of course, that this will happen again this year, but I would expect it to. The C&D singalong was a twilight type of activity and of course you need nightfall to see a movie well outside.