Conversation so far:
who did our lady of knock appear to
First to see the vision of Our Lady of Knock were Mary Beirne and Mary McLoughlin, housekeeper to Knock parish priest Archdeacon Bartholomew Cavanagh.
how many people witnessed the apparition of our lady of knock
In the village of Knock, 15 local people saw the apparition of Mary, St Joseph, St John the Evangelist, and a lamb standing on an altar before a cross

Question: when did the apparition happen
Self-contained rewrite: when did the apparition of our lady of knock happen