The Battle of Hogwarts was the final battle of the Second Wizarding War. It took place on May 2, 1998, within the castle and on the grounds of Hogwarts School of Witchcraft and Wizardry. When Lord Voldemort learned that Harry Potter was in the castle to locate and destroy one of his final Horcruxes, he ordered every Death Eater and creature that had pledged loyalty to him to attack the school. Dumbledore's Army communicated the need to fight to the Order of the Phoenix and their other allies, leading to a large-scale battle. Lord Voldemort personally led his forces in battle while Kingsley Shacklebolt and Minerva McGonagall led the defenders of Hogwarts. The battle ended with a decisive victory for the Order and the D.A., with many Death Eaters and Voldemort himself dead. It was the most devastating battle of the war, with casualties including Lord Voldemort and Bellatrix Lestrange, but also unfortunately Remus Lupin, Nymphadora Tonks, Severus Snape, Fred Weasley, underage Colin Creevey and at least fifty more who fought for Harry.
It should be noted that in the book, hundreds of underage students (under 17) were mandatorily evcuated through the Room of Requirement tunnel leading to the Hogs Head in Hogsmeade prior to the battle which started at midnight as ordered by Voldemort.