2nd year when Hermione asked Binns to tell them about the Chamber of Secrets. I know this because in five years of classes with Binns this was the one lesson where Binns had the whole class hanging on his every word for about ten minutes. Binns the only ghost professor at Hogwarts normally just reads his notes none stop in class and has a hypnotic voice that make even Goblin rebellions and Giant wars sound as boring as watching paint dry.