I find it fascinating and terrifying that on the one hand, these events were "close calls", on the other hand, considering that we can celebrate this event today, they were inevitable.
Logically speaking, we always must live in a world where past existential threats to humanity were avoided (including super-volcanoes, asteroids), but that says nothing about the possibility of future existential threats wiping out humanity.
Spoiler (including why it's related to your post) since I think it's easy to miss what this is: Vg'f n obk gung, jura bcrarq, qrfgeblf gur havirefr. Fvapr nccneragyl guvf obk yvirf va n znal-jbeyqf havirefr, vg'f onfvpnyyl n havirefr-fpnyr Dhnaghz Fhvpvqr qrivpr.
And yes, I'm extrapolating a bit, but given the rest of the SCP-universe (inasmuch as there is a such a thing), it's a pretty reasonable conclusion. If it's wrong to you, well, it's right to me, Hail Fanon, All Hail Discordia.
A war would have thoroughly wrecked civilization, but plenty of people would have survived, especially in third-world countries.
There's no reason a much diminished humanity concentrated in the southern hemisphere couldn't be marking the thirtieth anniversary of War Day today, if things had gone a bit differently.
Indeed. If you want to find out more, the best book that I've found on this subject is _Global Catastrophic Risks_, edited by Nick Bostrom and Milan M. Cirkovic:
"In Global Catastrophic Risks 25 leading experts look at the gravest risks facing humanity in the 21st century, including asteroid impacts, gamma-ray bursts, Earth-based natural catastrophes, nuclear war, terrorism, global warming, biological weapons, totalitarianism, advanced nanotechnology, general artificial intelligence, and social collapse. The book also addresses over-arching issues - policy responses and methods for predicting and managing catastrophes. "
FYI, if you call it Petrov day, your eastern european orthodox friends (such as Bulgarians and Russians) will tell you you are 3 months late. "Petrovden" (den == day) is the shorthand name of the St Peter's and Paul's day on the 29th June, it is a nameday of everyone called Peter, Paul and their variations, and it is very well recognized and celebrated.
Never heard of it, leave alone seeing anyone celebrating it.
Though I did hear about the "Saint Mother Mary of Egypt" day which was about every second Friday if you were to believe a heavy-drinking madam two floors down from where we lived.
The context is that it's the oldest inside Russian joke to make up random occassions when drinking. You feel like having a drink, but can't justify it - no problem, drink to the revitalizing power of parsley or to bid spring farewell or something as silly.
An interesting part of the article is that he was actually demoted after averting nuclear disaster. This is because admitting that there was a fault in the nuclear defense system would be tantamount to admitting that top Soviet military and scientific minds were capable of error. Rather than embarrass the regime, the military quietly moved Petrov out of the spotlight.
Command and Control[1] offers a comprehensive compendium of nuclear incidents if you're interested in more. It's quite remarkable that the US never faced an accidental nuclear detonation, despite the SAC, Air Force and Navy's bickering between themselves over control of them, and their refusal to fit safety devices until late in the war.
My favourite anecdote is the code on all warheads on the US' Minuteman fleet being set to 00000000 as an exercise in simultaneously conforming to and circumventing requirements for PALs (locks to forbid misuse).
The book "One Minute to Midnight" about the Cuban Missile Crisis has a rather long list of places during that confrontation that almost led to nuclear war.
One fascinating one was a U2 getting lost on the way to the North Pole due to the Northern Lights and flying back into the Soviet Union just at the wrong time - alarming the Soviets and resulting in fighters being sent up by the US to protect the ailing U2. Unfortunately, due to the US being at Defcon 2 the fighters were only armed with nuclear air-to-air missiles.... The relief (for a number of reasons) the U2 pilot felt when he eventually landed was, understandably, rather intense!
Logically speaking, we always must live in a world where past existential threats to humanity were avoided (including super-volcanoes, asteroids), but that says nothing about the possibility of future existential threats wiping out humanity.