"Voldemort is going to kill my mum and dad, and there’s nothing I can do to stop him.... I’m letting it happen... of course I have to watch."
-- Harry Potter (CC4.12)
31 October 1981
Harry Potter, his friends and family stand together while Lord Voldemort kills Harry’s parents and tries to kill baby Harry. The harsh green light of the Killing Curse floods the scene three times (CC4.12).
Hagrid arrives and goes into the Potter’s ruined cottage. He leaves flowers for James and Lily. He knows he cannot stay as “….them Muggles are coming…. with their flashing blues“. He greets baby Harry, promises to be his friend and then takes him away, wrapping him in the blanket (CC4.13).
All Hallows Eve - 31st October 1981
This event takes place at the end of the Voldemort Final timeline, as described in the essay "Albus Potter and the lesson in Quantum Mechanics" by Nick Moline.
His family: Ginny and son Albus; his friends: Hermione and Ron, as well as Draco Malfoy and his son's best friend Scorpius, all stay with Harry to watch Lord Voldemort at the Potter's cottage. Before now Harry was only able to hear their deaths happening via real (or Boggart) Dementors (PA4, PA10, PA12). The deaths of Lily and James are not any less horrible, even though everyone already knows what is going to take place (CC4.12).
In the end, as Hagrid leaves with baby Harry, they have come full circle, and the world is restored in Time (CC4.13).
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 31 July 2016: Harry Potter and the Cursed Child, Parts One and Two