The Pirates Expo is a non-canon event said to occur at various points over the years, where pirates come together and interact. A special Pirates Expo was held in the film One Piece: Stampede as part of Douglas Bullet's plot to become Pirate King.
According to Boa Hancock, Pirates Expos happen every few years, with no real established interval between them. Due to the massive number of pirates in close proximity to each other, the Expo is said to be a lawless and dangerous event.
A special Pirates Expo was planned by Douglas Bullet and Buena Festa. They invited the world's most notable pirates to Delta Island to engage in a hunt for Gol D. Roger's treasure, which is an Eternal Pose pointing to Raftel. However, the event was a trap designed for Bullet to kill all the pirates in attendance as part of his quest to become Pirate King. Bullet and Festa also arranged for the Marines to unleash a Buster Call on Delta Island during the Expo in order for Bullet to kill the Marine Admirals as well.