On the 30th day of Spring. Kai will visit your farmhouse. To celebrate the opening of the Beach Shack (and his return to the valley for the Summer), he invites you to come to the beach on the 1st day of Summer between 1pm and 6pm.
If you go to the beach on the 1st of Summer, several townsfolk from both the Forget-Me-Not Valley and Mineral Town will be there to celebrate. Kai will be cooking up food for all the attendees. There is no reward for attending the festival.