the day of the pizza is a historical fact for bitcoin, it was the first time it was used as a means of payment without an intermediaries! I hope that soon comes that day, I want to celebrate eating pizza!
You can do right now as bitcoin price is $7,900 
Yes, that is the moment that will not be forgotten for that person because he spent too much bitcoin for just buying pizza.
If he knows that bitcoin will be worth like today, I am sure he will not buying pizza using bitcoin, and he will save the amount for the pizza in his wallet.
But that happened in the past, he can remember for what he's done in the past, and he can tell the story for his kid or his grandson.
this comment and the usual statement mentioning "most expensive pizza" is a clear indication of the major differences between people's view towards bitcoin.
in early years (like the pizza time!) people weren't thinking of bitcoin as an investment to become rich by holding it. they were instead seeing bitcoin as a currency, a decentralized one that changed their lives. and that is why 10k bitcoin was spent that easily.