So, that is what the "big event" was while we were there. USO closed very early (6 p.m.?) and they were setting up some very elaborate entertainment venues. In fact, the DW was very excited to catch Alecia Keys rehearsing, along with two other (I guess) name acts. I didn't recognize any of them, being a Bluegrass fan.
They had very comfy-looking living room style chairs set up on the greens near the main stage, more stages set up throughout the park, and they were setting up wine bars near the Transformers back area (near Mel's). They also appeared to be setting up open bars near the main stage too. The next day we asked the Team Member at the boat landing (while waiting for a return trip to Portofino) what happened the previous evening. She stated that it was an appreciation event for Citywalk Employees. She said that Universal was a very good company to work for. My wife agreed with her on this, as her employer never shut down the business early and set up bars and top-name entertainment for them!