When did Alison die on Pretty Little Liars?

Answer 1

Well alison didn't die or pretended she killed her twin sister and took revenge
Alison doesn't really die it is her twin sister Courtney.

Alison was sent away to a mental institution because they thought she was Courtney. While Alison was away Courtney was pretending to be Alison because she had a very popular life at school. When she became friends with the four girls she found out all of their secrets and wrote them down in a diary. When the real Alison came back She killed Alison by Strangling her and put her in a hole. So then Mona became -A and knew all of the girl's secrets because she found the book that Courtney wrote. But then Mona dies because spencer finds out that she is -A and Mona tries to push her off a cliff but then Mona falls off and dies. Then the real Alison becomes -A. But the girls find out about Alison. But the real Alison apparently dies from a fire.

Hope I helped :)