When the Event is active, you can trade 3 Minnows for a Mistery Box by speaking with any Fisherman.
You can get Minnows by fishing, and you can trade them with the Fisherman as many times as you want, as long as the Event is active.
You need Level 15 to open the boxes.
Reward: Mistery Box
The contents of the Mistery Box are:
Our Event Calendar can be found here: Official SG2Global Event Calendar