"Well--there's a sort of riot going on. At the campsite...some people have got hold of a family of Muggles..."
-- Ron Weasley (GF9)
Sometime after the end of the Quidditch World Cup final, the carousing in the campground turns malicious.
A crowd of Death Eaters magically hoists the Muggle Roberts family high into the air and tosses them about. Other witches and wizards, perhaps drunk or merely overcome by a mob mentality, happily join in. Others, however, are terrified by the Death Eaters and revelers tormenting Muggles and casting blasting spells all over the place. Things come to a head when Barty Crouch Jr. casts the Dark Mark.
Mr. Weasley sends the underage members of his party to the woods while he and his of-age sons help the other ministry employees try to get control of the pandemonium. In the woods, Harry, Ron, and Hermione get separated from Ginny and the twins. Looking for them, the trio moves deeper and deeper into the forest, encountering Draco Malfoy (who practically admits his father is a Death Eater and taunts Hermione for being in extra danger due to her blood status), worried Beauxbatons students, some goblins more concerned with gold than with whatever troubles wizards are having, and three young men enraptured by Veela before crossing paths with Winky and Barty Crouch Jr. (although Winky is the only one who knows Crouch is there, underneath an invisibility cloak). The trio meets Ludo Bagman, who disapparates after they tell him what is happening at the campground. They then hear Barty Crouch Jr.’s voice as he conjures the Dark Mark. People are understandably terrified to see Voldemort’s symbol, and the Death Eaters, not wanting to be caught, take off.
The Quidditch final happened after dark, which would be starting around 10 pm. Assuming that the match lasted for at least a couple of hours, we can guess that Harry and the others didn't get back to their tent until at least 1am and didn't get to bed before 2 am (and probably later). The chaos started at least an hour after they went to sleep, so we'll say that it started around 3 am on the 26th.