From Bulbapedia, the community-driven Pokémon encyclopedia.
The Twinleaf Festival (Japanese: フタバ祭り Futaba Festival) is an event in Twinleaf Town that is held every year. It got a brief mention in Barry's Busting Out All Over!.
In Three Sides to Every Story!, it was revealed that Johanna was elected to be the chairperson of the most recent festival. She along with Izzy and Cara had organized the events and stalls. Professor Oak also came as a special guest for the festival in order to talk about Evolution and evolutionary stones in A Faux Oak Finish!.
The festival had many different contest booths, one of which was given to Brock and one to the Team Rocket trio; in Historical Mystery Tour!, a rivalry between Team Rocket's Goldeen Scoop Game and Brock's stall occurred due to Team Rocket's being much more successful.
The festival was also host to the tournament called the Festival Battle Challenge, from which the winner got the opportunity to battle Tower Tycoon Palmer. Ash, Dawn, Brock and Barry all took part. Ultimately, Ash won, and was given the chance to fight Palmer. In Challenging A Towering Figure, Ash used his Grotle, but lost to Palmer's Rhyperior.
There was also a singing competition at the end of the festival in which Barry took part.