The NPCs will somewhat explain, but this event is really just 1 FATE. You go out anywhere and look for the FATE "Return of the King". It starts at level 5, and there are versions every 5 levels it seems. The FATE is pretty easy, you just defeat small bombs while some friendly NPCs hit a boss bomb. You have to keep the friendlies alive long enough for them to fill the progress bar.

Anyway, completing the FATE will give you 30 Wet Bomb Ash which is the currency for the event. The vendors in town will sell the swimsuit and fireworks for these (and 1-2 FATEs are enough to buy everything).

Sorry, I don't know about any future events. There isn't anything currently that deals with lightning though.