It's a yearly tradition for people to descend on Embarcadero Plaza in San Francisco for a pillow fight at 6 PM on Valentine's Day. It's been going on for more than a decade and it will likely continue in 2020.
The event is unofficial and there could be hundreds of people, or just a few. It could go for an hour, or the police could shut it down before it starts, just be ready for each scenario if you're planning to attend.
It's advised that you bring a bandana, or mask to cover your mouth due to flying feathers.