How Many "Once-In-A-Lifetime" Days Can One Year Have?

According to the textbook, Thursday should almost never happen.

A move of 4.9 standard deviations in the Nasdaq, which is how Goldman's Mark Wilson describes Thursday's Nasdaq 100 crash, should come along about once every 8,000 years if returns were normally distributed (napkin math, but you get the idea). Instead, it was the latest entry in a year that has already given us the worst Nasdaq day since Covid, the biggest put day in Cboe history and a VVIX move topped only by Volmageddon.