aug 1, 1971 - The Whomping Willow is planted at Hogwarts

Description:

The tree is magically enchanted to protect what is concealed in its roots: the entrance to a secret passage to the Shrieking Shack. It is planted so that when Lupin arrives as a student, he can be taken by Madam Pomfey to the Shrieking Shack once a month when he transforms into a werewolf.

The connection between the Whomping Willow and the Shrieking Shack is kept secret for the next two decades — even Fred and George, who could see the secret passage on the Marauder’s Map, assumed that no one had used the passage because the Whomping Willow was planted right on top of it

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