The Swallowtail Festival serves to commemorate a legend in which an avatar of Desna fell to Golarion after a fierce battle with the goddess Lamashtu. She was discovered and nursed back to health by a blind orphan. To express her thanks, Desna transformed the child into a beautiful, immortal swallowtail butterfly so that he could explore the wonders of the whole world for all eternity.

To celebrate the festival, priests of Desna release a wagon full of swallowtail butterflies in front of a crowd of believers. Worshipers consider it a good omen if a butterfly rests on him or her for a short time. The remainder of the day is spent in celebration and can include activities such as feasting, storytelling, and singing. In preparation for this event, larger temples of Desna often have enclosed gardens in which they raise swallowtail butterflies year round. In colder climes, where the raising of butterflies is impossible, faithful will sometimes substitute colored leaves or painted corn husks for the actual insects.