I introduced this to my old company years ago and called it Big Block of Cheese Day after the West Wing episode [1]. We mostly focused on very minor bugs that affected a tiny bit of our user base in edgey edge cases but littered our error logs. (This was years ago at a, back then, relatively immature tech company.)
It had the same spirit as a hackathon.
[1] https://westwing.fandom.com/wiki/Big_Block_of_Cheese_Day