"My subject is History of Magic. I deal with facts, Miss Granger, not myths and legends."
-- Professor Binns hoping to deflect Hermione's question about the Chamber of Secrets (CS9)
Professor Binns is perplexed when his students actually start discussing something in his class, particularly because the topic — the Chamber of Secrets — is something which he considers to be purely a legend and therefore not worthy of his time in a facts-based class.
This event happened the first Wednesday after Halloween, just after lunch, in History of Magic class. In 1992, that was November 4.