"I don’t know what game you’re playing but you’re upsetting the Dementors and entirely ruining Voldemort Day."
-- Dolores Umbridge (CC2.20)
In an alternate timeline where Harry Potter died at Battle of Hogwarts (arising from the use by Albus Potter and Scorpius Malfoy of the Experimental Time Turner), this day for the celebration of Lord Voldemort features giant green banners with snake symbols hung around Hogwarts School (CC2.20).
Scorpius is shocked to find himself in a world where Voldemort Day exists. Trying to find out what has happened gets him into trouble with Hogwarts' Headteacher, Dolores Umbridge (CC3.1).
From the Web
About the play: http://www.harrypottertheplay.com