The Battle of Hogwarts took place on May 2, 1998. The venue was the grounds of Hogwarts, as well as the castle interior.
On his quest to destroy all the Horcruxes of Voldemort, Harry goes to Hogwarts to find the final ones. Voldemort learned of this, and ordered an attack on the school. Everyone who were under the orders of Voldemort were sent there. On the other hand, Harry was well braced. Dumbeldore's brother contacted everyone from Oliver wood to the Weasly twins (including Percy-who want his family to forgive him)