I found a map at dusk:
At one point, while playing on Risa, I happened on a map that was at dusk. How to tell it's dusk? Check the suns/moons. Unlike Earth, Risa's rotation is very odd... The suns set in the South (rise in the North). The moons rise in the Southeast (set in the West). So if you see these astral bodies to the South, and it's getting dark. Dusk. Also, the Resort starts to light up and set bonfires.
The overall time from when I first happened into the "dusk map" and I got the accolade was maybe 20 minutes. During which time I was doing "other things" (including one time jumping to another map and back), but I kept watching the settings suns...
I took up position on the beach:
When the first sun touched the horizon, I got in position. Someone had earlier shown me where they got their accolade, and it corresponds pretty well to point H on Smiecho's Map. I doubt you'd have to be here this soon, but I didn't want to miss it, and didn't know what to expect...
What Exactly Happened:
I was standing in position, facing Southwest. As the second sun set, a bunch of NPC Vactioners faded-in. Most of them were sitting cross-legged on the beach where I was standing. I noticed many others also now standing/sitting around the beach and the bamboo huts in the water to my left (East of the Floater shop). They were all facing to the Southwest, looking up into the sky.
The sky continued to darken and stars began to appear (gradually fading-in). This took a couple minutes. As it really became dark and both moons had risen above the horizon, the show started. It took up most of the sky in my view. The fireworks themselves were scaled up animations/sound effects from the Veteran Fireworks device. Maybe 30-45 seconds after the pyrotechnic show started, the accolade triggered. The show proceeded for a few more seconds, then ended.
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.