Always seeking to arouse curiosity in his grandson, Grandpa has another challenge for him:
Grandpa: Say, Billy, have you heard of the Event?
Billy: What event Grandpa?
Grandpa: Well, the Event comes back more or less regularly. George Washington witnessed it once, as did Napoleon, Genghis Khan and Sir Isaac Newton, but neither Leonardo da Vinci, Charles I of England, Pope Innocent III nor William Shakespeare ever did.
Billy: So it's an event one can see only once or never in its life?
Grandpa: Not at all! In fact, I witnessed it twice!
Billy: Wow, that's cool! And what about me?
Grandpa: You're only ten, so...
Billy: Ten and a half.
Grandpa: ...Ten and a half then, but it makes no difference, you're too young to have experienced the last Event. However, if you live long enough, you just might see the next one.
Billy: Aww... and what about other people from our time?
Grandpa: Tobey Maguire and Andrew Garfield lived through two Events, but Tom Holland only saw one. John Lennon witnessed no Event at all, while Norman Chapman saw one and Paul McCartney two.
Billy: Did someone ever saw more than two?
Grandpa: Absolutely! Take Charlemagne for example: he witnessed no less than six Events, and probably seven!
Billy: Wow! And who saw the most?
Grandpa: Well, probably an anonymous nobody living a long time ago. But among the personalities we know of, let's see... hum, I'd say maybe the roman emperor Galba, who should have lived through fifteen of them! Maybe someone got up to 16, 17 or even 18 - and at that point 19 is not a lot less believable than 18 - but I seriously doubt someone lived 20 of them, or more.
Billy: OK Grandpa, but I still have no idea what this event of yours is.
Grandpa: Then let's ask your friends at stackexchange, maybe they'll find out ;).
What is the Event, and how many did you personnally witness? (Anyone saying more than 2 is a liar)