"….the lesson even your father sometimes failed to heed is that bravery doesn’t forgive stupidity. Always think. Think what's possible."
-- Minerva McGonagall (CC3.10)
An extremely angry and distressed Headmistress McGonagall gives Albus Potter and Scorpius Malfoy detention for the rest of the year. She points out the stupidity of what they were attempting and, even though their intentions were “honourable, but misguided”. Their actions had “resurrected the worst wizard the world has ever known and heralded in a new age of Dark Magic” (CC3.10).
Directly after the previous event, when Albus and Scorpius return from the first two uses of the Experimental Time-Turner - likely early October 2020
This event takes place in the Repaired timeline, as described in the essay "Albus Potter and the lesson in Quantum Mechanics" by Nick Moline.
Minerva McGonagall mentions that they are all too young to remember the "wizarding wars" and the sacrifices that people made so they could live without fear (CC3.10). The "wizarding war" she is referring to is the rise of Voldemort in the 1970s and 80s, before he was vanquished trying to kill the baby Harry Potter on 31 October 1981.
From the Web
Pottermore: Harry Potter and the Cursed Child
The theatre production opened in the West End of London at the Palace Theatre on 30 July 2016, with Anthony Boyle in the role of Scorpius Malfoy, Sam Clemmett as Albus Potter and Noma Dumezweni as Hermione Granger: Harry Potter and the Cursed Child, Parts One and Two