Alt #1 was in the middle of the Dance Party! mission, I believe he was at the x/10 portion of the dance. I heard Fireworks, turned around, and saw Fireworks. I ran off the dance floor over to the Point H mentioned above. Stood there for 10 seconds or so, and I got the accolade. I then ran back and resumed/finished my Dancing (and got Advanced Samba).
Alt #2 was completely by accident. I was in the Resort, working on the Exchange (the East terminal). I was not at, near, or otherwise paying any attention to the show (I'm not even sure if I heard it happening). But as I was closing up the Exchange, the Firework Observer accolade dinged. That was cool.