Terraria Community Forums

The event starts the moment you kill the first cultist. And continues until you kill the final boss. Being killed at any point after you kill the first cultist means you have to wait one Terraria day for them to respawn and try again.

But,do most people fight Duke Fishron first, or attempt the newer event?

When I get close to the endgame in my Expert Mode playthrough, I will likely do Fishron either before or after beating Plantera because I struggle against Golem without Fishron weapons. Even on Normal Mode, Fishron has to be beaten in the Ocean now, even if its extremely difficult I will need its gear to have even a shot at the Lunar Event which is even harder than Fishron in both difficulties.