From the organizer site:
Established in 2007, Free RPG Day works with participating hobby game retailers and RPG publishers to bring new and exclusive RPG quickstart rules and adventure modules into the hands of gamers.
Consumers worldwide will be able to grab brand new material for a variety of RPGs - no overstock, retail-priced or dead product here. The goal of Free RPG Day is to inspire gamers to play a new RPG, which will in turn, create sales through local game stores.
Free RPG Day 2018 is Saturday, June 16, 2018.