From the original event description:

On September 26th, 1983, the world was nearly destroyed by nuclear war. That day is Petrov Day, named for the man who averted it. Petrov Day is now a yearly event on September 26 commemorating the anniversary of the Petrov incident.

The purpose of the ritual is to make catastrophic and existential risk emotionally salient, by putting it into historical context and providing positive and negative examples of how it has been handled. This is not for the faint of heart and not for the uninitiated; it is aimed at those who already know what catastrophic and existential risk is, have some background knowledge of what those risks are, and believe (at least on an abstract level) that preventing those risks from coming to pass is important.

We'll be hosting this ritual in Cambridge at Autumnfell (unit 2 of the above address). It will begin at 8:00 and last for about an hour. Please do not arrive in the middle.