Bubble Fest is an event that was held in the episode Pop Goes the Bubble. It is a party with a bunch of bubbles in it.
The Bubble Fest features all kinds of bubble related entertainment and commodities. The most common drink available is Pop.
In Pop Goes the Bubble, Grojband are hired to perform a gig at the festival. However Trina ruins Grojband's chances using a bottle of overshaken pop, that lodges a cap in Mayor Mellow's brain and causes him to hate bubbles. The Mayor proceeds to ban the bubbles and the festival. However Grojband works behind the scenes to return the bubbles into the pop and restores the festival to its glory. In the end they perform their song This Bubble Don't Pop which is a huge hit.