The mystery of July Third

Every night on a July third Paris would kidnap a young girl and force her to pick between switching souls or meeting her demise.

When the girl recites the words as she's told and nothing happens after, Paris would murder her.

July third is here again and Paris kidnaps Astrid and forces her to repeat the same words, when Astrid does and the switch actually takes place Paris is left to wonder. Apart from a bully and her victim, what is the connection between she and Astrid? Why does she want to snap her neck in two every time she lays eyes on her? Who exactly placed this curse on her and why? Most importantly what happened on July third?