SS/PS: The troll is let in so Quirrell can go try to get past Fluffy.
CoS: Harry hears the basillisk saying the "rip tear kill stuff"
PoA: Sirius Black gets into Hogwarts
GoF: The champions are chosen, including Harry
OoTP: I haven't really looked yet so if anyone can find it that would save me some trouble
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Also, isn't Halloween the day Voldemort came into Godric's Hollow and murdered Harry's parents and Harry got rid of Voldy? I always wondered why Harry wasn't all extra-sad and depressed on Halloween seeing as it's the anniversary of his parents death, and why people aren't all congradulatory towards him because its also the anniversary of Voldy going away. McGonnigal said in SS/PS that "today will probably be called Harry Potter Day" but it never was. Maybe after 11 years of Voldy being gone people didn't even want to talk about anything connected with Voldy because of the whole superstitious mentioning-the-name-thing.
Ok I don't really have a theory about all this, I just thought it was very interesting so I hope I haven't stated the obvious for any of you!
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But I do think Halloween is a very important symbol in HP seeing as it's when Harry's parents were killed and Voldy almost died.