It's The Great Pumpkin, Harry Potter!

Professor McGonagall enters the Gryffindor Common Room to a sight that can set anyone’s heart a-flutter, or turn one’s mind to carnal thoughts. But what is she doing in the Common Room in the first place – and on the day after Halloween? This is one of my earliest fanfics which I had almost totally forgotten about, until a gentle reminder from briarswt made me look at it again. I'm re-posting this at portkey; I hope you guys enjoy!