Ahoy Maties!! Season 5 brought us a new commendation section in the Bilge Rats portion of your journal (Buried Treasures), and a lot of achievements tied to these commendations.
This particular achievement tasks you with setting of a display of *five* or more fireworks at night. While the xbox description says 3 or more, the achievement is tied to the in-game commendation, which requires five or more to be shot at night. This is a crew based achievement and shots from crewmates will count towards this as you all are launching fireworks.
Fireworks can be found in barrels on all islands, you can find firework crates washed up on beaches, and you can also buy firework crates from the merchant alliance (for a heft 20,000 gold).
I have also created a video guide if you prefer video! See you on the Seas!
-Damron Out