Final answer:
The event rumored to be the only occasion on which Philip II was known to have smiled was the St. Bartholomew's Day massacre, a violent massacre of Huguenots in France during the ongoing conflict between Protestants and Catholics.
Step-by-step explanation:
The event rumored to be the only occasion on which Philip II was known to have smiled was the St. Bartholomew's Day massacre. This event took place on August 24, 1572, in France during the ongoing conflict between Huguenots (Protestants) and Catholics. It was a violent massacre of Huguenots who had come to Paris for a royal wedding, orchestrated by the French Catholic troops. Peace was eventually restored in France when the Huguenot Henry IV converted to Catholicism and issued the Edict of Nantes in 1598.