The Steam Game Festival debuted in December 2019 in conjunction with The Game Awards, with time-limited demos for 13 upcoming games. A spring edition followed in March, to help make up for the loss of GDC, with a longer time-frame—five days instead of 48 hours—and demos for 40 games. The a week-long Summer Game Festival dropped 900 demos in our lap and, well, we did our best.
And it's coming back again: Geoff Keighley, who knows about these sorts of things, said on Twitter today that the Steam Game Festival: Autum Edition will give us even more game demos in October.
Valve is planning another Steam Game Festival: Autumn Edition this Fall, running October 7 - 13. Another chance to play free previews of upcoming games.August 11, 2020
Valve hasn't revealed any details about the fall festival just yet, but I sincerely hope that it takes a different approach than the summer event. 900 game demos all at once was way too many, especially for an event that only ran for a week. There's just no way to get through more than a small fraction of that number in any useful way in such a short time-frame, which means that attention ends up going to games that are already well known and thus least in need of it. A week of free stuff is great, but as a way to discover new and interesting things that you might've overlooked, I don't think it's a good approach to take.
I've reached out to Valve for more information on the next Steam Game Festival, and will update when I know more.