Petrov Day is a tradition celebrating Soviet military officer Stanislav Petrov, who played a key role in preventing a nuclear attack during a false alarm incident on September 26, 1983.
petrov.com has an organizer's guide for a small quiet ceremony, and Modes of Petrov Day contains additional ideas. Otherwise see the many posts tagged Petrov Day for things people have done in various times/places/groups.
The LessWrong site has an annual tradition of commemorating the day involving users having the potential to take down the LessWrong frontpage for 24 hours. This has happened since 2019. Here are the relevant posts....