This is evident from a message on Steamworks , the developer environment of the PC platform. Steam is calling on developers to sign up their demo for the upcoming event.
You are not allowed to submit demos that were available during the two most recent editions, like the one from earlier this month, and the lineup focuses on titles coming in the second half of 2021.
During Steam Game Festivals, users can download free demo versions of upcoming titles, ask questions to developers, and provide feedback. A jury at Steam will determine which games will be featured based on the store page and trailers.