The event is live today and is set to run until the release of Patch. 5.4. New weapons and minions are set to make their debut this time around, including a new mount. Because of this, the number of Yo-Kai medals needed for weapons and minions was reduced.
If you want to take part in the event, be sure to chat with the Poor-healed Youth (seriously) in Ul’dah. If you already completed this quest, however, no worries. Just start collecting the medals. You can do this by equipping your Yo-kai Watch. You can buy this watch from the Calamity salvager in each city-state.
As with any event, there are rewards up for grabs. These rewards include the Gunblade of the Yo-kai King / Minion: Lord Enma, Rapier of the Serpent Lord / Minion: Lord Ananta, Katana of the King's Counsel / Minion: Zazel, Glaives of the Dark Princess / Minion: Damona, and Whisper-go / Whisper A-go-go / Jibanyan Couch Mount.