Earlier this month the Steam Game Festival's week-long winter event brought over 500 new demos to Steam. Fortunately some developers took Valve up on the option to have demos remain available once the week ended, and you can still find playable slices of games like Potion Craft: Alchemist Simulator, Loop Hero, Warhammer 40,000: Dakka Squadron, and Little Nightmares 2 on their Steam pages.
The follow-up summer event will take place between June 16 and June 22, as noted on the ResetEra forums. Once again there will be demos, livestreams, and developer chats in support of upcoming games. Games being considered for the event have to be unreleased, have a release date of no later than January 2022, and have not participated in either of the previous two Steam Game Festivals. (Early Access games will be considered as long as they haven't released a playable build before the festival begins.)
While Steam has sometimes run spring sales in the past, we've gone two years without one now. If that trend continues, the Steam Game Festival's summer edition may well coincide with the next big Steam sale.