"Thanks to Delphi’s brilliant preparation work, we are going to take this potion and be transformed, and thus disguised we will be able to enter the Ministry of Magic."
-- Albus Potter (CC1.16)
Delphi produces Polyjuice Potion for her, Albus Potter and Scorpius Malfoy to take, in order to disguise themselves and get into the Ministry of Magic headquarters. Delphi becomes Hermione Granger, Albus is his uncle, Ron Weasley, and Scorpius transforms into Harry Potter. They enter the Ministry via the telephone box (CC1.16).
This is the day after Albus and Scorpius escape from the Hogwarts Express and visit the St Oswald's Home - so during 2 September 2020.
It is suspiciously convenient that Delphi has Polyjuice Potion ready to be taken so quickly. It needs at least three weeks for the Lacewing Flies to be properly stewed. However, there is a theory that the ingredients may be available on Diagon Alley to purchase.
From the Web
The theatre production opened in the West End of London at the Palace Theatre on 30 July 2016, with Esther Smith in the role of Delphi, Anthony Boyle as Scorpius Malfoy and Sam Clemmett as Albus Potter: Harry Potter and the Cursed Child, Parts One and Two