The Steam Spring Cleaning Event kicked off today, and as the name suggests it's not actually a sale. Instead, it's a way to catch up with your backlog, and earn a new Steam badge in the process, by completing daily tasks with games you already own.
Part one is to earn "trophies" in five different categories: Play a game you've got more than two hours into "but haven't played in awhile," for instance, or play the very first game added to your account, excluding Half-Life 2 if you go back that far. The games are selected for you and are linked from the Spring Cleaning page, so it's easy to pick a relevant game.
Once you've got all five trophies in place, the badge is yours, at which point you can get to work completing daily tasks—playing games selected from your library by Valve, basically—in order to level it up.
A number of games have been made free-to-play for the weekend too, including Dead by Daylight, Endless Space 2, Grim Dawn, Don't Starve Together, Black Desert Online Remastered, Assetto Corsa, and Left 4 Dead 2. That's a nice bonus for people who don't really care about Steam badges but wouldn't mind having something new to try this weekend.
The Steam Spring Cleaning Event runs until 10 am PT on May 28.