The Milkberry Harvest Festival is a celebration that takes place during the season of the copper rose at The Great Ga'Hoole Tree. It is said that at this time the milkberries are at their most ripe, and owls will go outside of their normal schedules in order to pick the berries.
Led by the Ga'Hoolology ryb of the Great Tree, the festival consists of berry picking, singing, drinking, and dancing. Chaw practices and classes are put on hold for three days during which the berry-picking portion of the festival takes place, then on the third night, there is an enormous banquet that continues each night for three or four nights. In the banquet, the rybs are said to often get very tipsy on milkberry wine. The dances named are the "Glaucana," described as sweeping from wing to wing in a "stately waltz", and the "Glauc-Glauc," vaguely described by Otulissa as having the steps "one two, glauc-glauc-glauc, then backward one two, glauc-glauc-glauc."
The festival is led by Dewlap, the Ga'Hoolology ryb at the time. The celebration begins with Madame Plonk and Dewlap singing the "Harvest Hymn" to the plucking of the harp played by the nest-maid snakes, before Bubo interrupts with a drinking song. During the festival, the Band, along with Eglantine, are planning to secretly escape to seek out the Rogue Smith of Silverveil to find information about Metal Beak and potentially learn about Ezylryb's disappearance. In order to do so, they must wait until everyone is too tipsy to notice them leave, and end up making their way out when the drunken owls were distracted by Madame Plonk, who had passed out, and the light of a comet passing by in the night.
The owls of the Great Tree simplify the Milkberry Harvest Festival in order to appease the Striga. The Striga objects to all things he considers "vanities", so because of the debt they owe him for saving Coryn and Soren from the assassination, they do away with the singing, dancing, decorations, and drinking. This distresses Blythe, who had been learning to sing with Madame Plonk, and was supposed to sing for the festival.
The book opens up with the next year's Harvest Festival celebration, Blythe singing the opening song for it, as well as some gadfeather tunes. She is said to have created a new, more modern style of music called R-and-H, which stands for "Rhythm and Hoots". Blythe also caused a change in the Harp Guild's style of plucking.