Snape “accidentally” lets it slip, the morning after Sirius Black escapes. However, Hagrid knew, and wondered why Harry had not heard about Lupin from Professor McGonagall (PA22).
Remus Lupin resigns from Hogwarts after it becomes known he is a werewolf
Date June 7th, 1994
Certainty Date based on real world calendar
Location in Canon PA22
Type of Event Wizarding world