On the evening of May 26th, 1992, Harry, Hermione, Draco, and Neville serve detention. The punishment, which they earned by trying to get Hagrid out of his own stupid mess with the baby dragon, is to tramp around the forest in the middle of the night, unarmed, looking for something ruthless enough to attack unicorns. To add to the fun, Hagrid splits them up into two groups.
Yes, Neville. Read the book.