The hostility towards Harry since he became the second Hogwarts champion – and Harry’s own frustration in response – comes to a head when Harry and Malfoy have a confrontation before Potions class (GF18).
Harry and Hermione head to Double Potions after lunch and find the Slytherins waiting outside the dungeon and wearing badges that say “SUPPORT CEDRIC DIGGORY – THE REAL HOGWARTS CHAMPION!” Malfoy and the other Slytherins taunt Harry by showing him that the badges can also display the message “POTTER STINKS”. When Malfoy also tells Hermione that he doesn’t want her to touch him because she is “a Mudblood,” Harry pulls out his wand. Malfoy quickly does likewise. They cast their spell at the same instant, and the spells bounce off each other – with the result that Harry’s spell gives Goyle some nasty boils and Malfoy’s spell causes Hermione’s front teeth to grow extremely long extremely quickly and painfully.
Snape arrives. He tells Goyle to go to the Hospital Wing, but cruelly says he doesn’t see any difference when Hermione’s even-larger-than-normal teeth are brought to his attention. Harry and Ron both start yelling at Snape after Hermione runs away humiliated. He gives them detention and takes fifty points from Gryffindor for each of them (GF18).
Note: The Furnunculus Curse, which Harry uses here, is called the “Pimple Jinx” in Pm. In this instance, however, the skin eruptions the spell causes are “great ugly boils” (GF18), not pimples.
We know GF18 begins on Sunday morning, the morning after the Hallow'een feast, which dates it as Sunday, 1 November 1994 [Y14]. This fight takes place on the second Friday after that, which puts it on Friday, 13 November. We also know that Double Potions was the class immediately after lunch on Fridays (GF18).