"...it is a monstrous thing, to slay a unicorn ... Only one who has nothing to lose, and everything to gain, would commit such a crime. The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenseless to save yourself, and you will have but a half-life, a cursed life, from the moment the blood touches your lips."
-- Firenze (PS15)
Harry and Draco are on detention in the Forbidden Forest along with Fang the Dog when they view a monstrous hooded creature beside a dead unicorn drinking the animal’s silvery blood. As the creature turns to stare and then crawls on the ground towards them, Draco screams and runs away with Fang, and Harry feels a burst of pain in his forehead scar. At that moment, the centaur named Firenze leaps over Harry and chases away the unicor’s killer, which was actually Professor Quirrell possessed by Lord Voldemort (PS15).
When the other centaurs arrive, Bane taunts Firenze for allowing Harry to ride on his back “like a common mule.” But Firenze allows Harry to ride back to meet Hagrid, while explaining to him the importance of unicorn blood and the fact that Voldemort might use it to return to human form (PS15).
There is no date given except that it takes place before final exams.